drm/radeon: Update AVIVO cursor coordinate origin before x/yorigin calculation.

Fixes cursor disappearing prematurely when moving off a top/left edge which
is not located at the desktop top/left edge.

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