drm/radeon: Set cursor x/y to 0 when x/yorigin > 0.

Apart from the obvious cleanup, this should make the line

			cursor_end = x - xorigin + w;

correct now.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>