drm/amd/display: Test GFX12 DCC plane attributes

drm/amd/display: Test GFX12 DCC plane attributes

[WHAT]
Add KUnit tests for
amdgpu_dm_plane_fill_gfx12_plane_attributes_from_modifiers() covering
the block0, unconstrained-block and validation-failure cases.

Add the dm_test_gfx12_attrs() helper used to build the GFX12 inputs.

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>