drm/amd/display: Add KUnit test for native backlight registration

[WHAT]
Add a KUnit case covering the successful path of
amdgpu_dm_register_backlight_device(). The test uses the standard DRM
KUnit device helpers to register a native backlight device, verifies the
calculated backlight properties and cached brightness, and unregisters
the device through KUnit cleanup to avoid leaking a class device across
cases.

Assisted-by: Copilot:GPT-5.6-Terra
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

0 files changed, 0 insertions, 0 deletions