drm/amd/display: Test plane atomic check and hooks

drm/amd/display: Test plane atomic check and hooks

[WHAT]
Add KUnit tests for amdgpu_dm_plane_atomic_async_check(),
amdgpu_dm_plane_atomic_check(), amdgpu_dm_plane_panic_flush() and the
DRM plane reset, duplicate-state and destroy-state hooks.

Add the dm_test_init_atomic_check_state() helper and plane reset vtable
used by the new tests.

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>