The core code for PREPARE_BUF didn't take the multiplanar case into account,
which might cause page faults. Handle PREPARE_BUF just like QUERYBUF and
QBUF/DQBUF.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>