Async plane update is supposed to work only when updating the FB or FB
position of an already enabled plane. That does not apply to requests
where the plane was previously disabled or assigned to a different
CTRC.

Check old_plane_state->crtc value to make sure async plane update is
allowed.

Fixes: fef9df8b ("drm/atomic: initial support for asynchronous plane update")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180724133215.31917-1-boris.brezillon@bootlin.com