drm/amd/display: Test DCC and cursor edge cases

drm/amd/display: Test DCC and cursor edge cases

[WHAT]
Add KUnit tests for the capability-callback-failure and not-capable
paths of amdgpu_dm_plane_validate_dcc(), and for the bad-size path of
amdgpu_dm_plane_get_cursor_position().

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>