drm/amd/display: Add KUnit test for replay

drm/amd/display: Add KUnit test for replay

Add KUnit tests for amdgpu_dm_link_supports_replay() which
validates panel replay capability based on link DPCD caps,
freesync state, and VSDB info. Nine test cases cover the
positive path and each individual failure condition.

Export the function under CONFIG_DRM_AMD_DC_KUNIT_TEST and
add the amdgpu include path to the tests Makefile so that
amdgpu_dm.h can resolve amdgpu_mode.h types under UML.

Assisted-by: Copilot:Claude-Opus-4.6
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>