drm/amd/display: Test GFX11/GFX12 plane modifiers

drm/amd/display: Test GFX11/GFX12 plane modifiers

[WHAT]
Add KUnit tests for amdgpu_dm_plane_get_plane_modifiers() on GFX11
(64K-first and 256K-first) and GFX12 devices.

Add a register-read mock (dm_test_gfx11_reg_ctx and friends) and
device-setup helpers so the GFX11 DCC modifier ordering can be checked
without real hardware.

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>