drm/amd/display: Test I2C, stubs and MST early returns

[WHAT]
Add KUnit coverage for dm_helpers_submit_i2c,
dm_helper_dmub_aux_transfer_sync, the empty stub helpers, the MST
null-connector early returns, dm_helpers_dmub_outbox_interrupt_control,
dm_helpers_mst_enable_stream_features and
dm_helpers_enable_periodic_detection.

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>

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