drm/amd/display: Extract backlight helpers for KUnit tests

[WHAT]
Extract shared backlight device index lookup and property
setup into testable helpers. The duplicated bd-to-index
scan in update_status/get_brightness is replaced by
amdgpu_dm_backlight_get_device_index(), and the inline
backlight_properties calculation is replaced by
amdgpu_dm_backlight_fill_props().

Add KUnit coverage for both new helpers. Keep the runtime
power_supply_is_system_supplied() call at the caller so
the helpers remain pure and deterministic under test.

Assisted-by: Copilot:GPT-5.5
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>

0 files changed, 0 insertions, 0 deletions