[WHAT]
Add KUnit coverage for dm_helpers_construct_old_payload, the DP MST
payload allocation, poll-for-ACT and deallocation success paths, the
full dm_helpers_dp_write_dsc_enable matrix and
dm_helpers_dp_handle_test_pattern_request.

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

[WHAT]
Add KUnit coverage for dm_helpers_dp_mst_start_top_mgr and
dm_helpers_dp_mst_stop_top_mgr, dm_helpers_dp_write_hblank_reduction,
get_dsc_max_slices, dm_helpers_init_panel_settings,
dm_helpers_override_panel_settings and
fill_dc_mst_payload_table_from_drm.

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

[WHAT]
Add KUnit coverage for DTN logging, DPCD read/write, fused IO and the
Synaptics DSC workaround helpers execute_synaptics_rc_command,
apply_synaptics_fifo_reset_wa,
write_dsc_enable_synaptics_non_virtual_dpcd_mst and
dm_helpers_dp_write_dsc_enable.

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

[WHAT]
Add KUnit coverage for the EDID parsing helpers edid_extract_panel_id,
apply_edid_quirks and dm_helpers_parse_edid_caps, together with the
ACPI/VBIOS EDID readers dm_helpers_probe_acpi_edid,
dm_helpers_read_acpi_edid and dm_helpers_read_vbios_hardcoded_edid.

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Add amdgpu_dm_helpers_test.c with 32 KUnit test cases covering the
following functions in amdgpu_dm_helpers.c:

- edid_extract_panel_id(): basic extraction with known mfg_id and
  prod_code; zero inputs produce zero output.
- dm_is_freesync_pcon_whitelist(): every entry in the whitelist
  table returns true; an unknown ID and a zero ID return false.
- populate_hdmi_info_from_connector(): scdc_present is copied from
  hdmi->scdc.supported for both true and false; FRL DSC fields map
  10bpc and 12bpc correctly and ignore unknown values.
- dm_get_adaptive_sync_support_type(): five cases covering the
  default non-converter path, HDMI converter without conditions,
  partial conditions, all conditions met with a whitelist device
  (FREESYNC_TYPE_PCON_IN_WHITELIST), and all conditions met with a
  non-whitelisted device.
- dm_helpers_is_fullscreen() / dm_helpers_is_hdr_on(): stubs always
  return false.
- get_max_frl_rate(): all six valid lane/rate combinations plus the
  unknown combination returning 0.
- dm_dtn_log_begin()/dm_dtn_log_append_v()/dm_dtn_log_end(): buffer
  accumulation and NULL-context handling without crashing.
- dm_helpers_dp_read_dpcd()/dm_helpers_dp_write_dpcd(): NULL link
  private data returns false.
- dm_helpers_dp_mst_start_top_mgr()/dm_helpers_dp_mst_stop_top_mgr():
  NULL link private data and the boot path.
- dm_helpers_dp_write_hblank_reduction(): stub returns false.

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Chenyu Chen <chen-yu.chen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>