drm/amd/display: Test GFX9/GFX10 plane modifiers

drm/amd/display: Test GFX9/GFX10 plane modifiers

[WHAT]
Add KUnit tests for amdgpu_dm_plane_get_plane_modifiers() on GFX9,
Raven, Raven constant-encode, GFX10.1 and GFX10.3 devices.

Introduce the dm_test_expect_mods_terminated(), dm_test_mods_contain()
and dm_test_get_primary_mods() helpers used to validate the generated
modifier lists.

Assisted-by: Copilot:Claude-Opus-4.8 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>