drm/ast: fixed cursor may disappear sometimes

commit 7989b9ee8bafe5cc625381dd0c3c4586de27ca26 upstream.

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>