drm/qxl: unref cursor bo when finished with it

qxl_cursor_atomic_update allocs a bo for the cursor that
it never frees up at the end of the function.

This commit fixes that.

Signed-off-by: Ray Strode <rstrode@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>