[media] vb2: fix plane index sanity check in vb2_plane_cookie()

It's also invalid when plane_no is equal to vb->num_planes

Signed-off-by: Zhaowei Yuan <zhaowei.yuan@samsung.com>
Cc: stable@vger.kernel.org      # for v3.7 and up
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>