drm/amd/display: disallow cropping for cursor plane

Looking at handle_cursor_update, it doesn't seem like src_{x,y,w,h}
are picked up by DC. I also tried to change these parameters via a
test KMS client, and amdgpu ignored them. Instead of displaying the
wrong result, reject the atomic commit.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>