drm/amd/display: Test GFX9 DCC plane attributes

drm/amd/display: Test GFX9 DCC plane attributes

[WHAT]
Add KUnit tests for
amdgpu_dm_plane_fill_gfx9_plane_attributes_from_modifiers() covering
DCC enablement, validation failure, and the RB+ 64B/128B and GFX9
64B/unconstrained block modes.

Add the dm_test_gfx9_attrs(), dm_test_setup_gfx9_dcc_device() and
dm_test_gfx9_dcc_modifier() helpers used to build the GFX9 DCC 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>