drm/amd/display: add basic atomic check for cursor plane

drm/amd/display: add basic atomic check for cursor plane

This patch adds a basic cursor check when an atomic test-only commit is
performed. The position and size of the cursor plane is checked.

This should fix user-space relying on atomic checks to assign buffers to
planes.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reported-by: Roman Gilg <subdiff@gmail.com>
References: https://github.com/emersion/libliftoff/issues/46
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <hwentlan@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>