drm/amd/display: Test backlight level readback path

drm/amd/display: Test backlight level readback path

[WHAT]
Add KUnit coverage for the backlight level read path.

A faked link_service vtable returns canned PWM and AUX brightness values,
letting the tests verify both the successful readback and the
DC_ERROR/AUX-failure fallbacks to the cached brightness, plus the
get_brightness() device-index wrapper.

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>