[media] v4l2-ioctl.c: handle PREPARE_BUF like QUERYBUF

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.

Change-Id: Ida7f712edec32568a8c2651d54b160121009a9c4
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Srinu Gorle <sgorle@codeaurora.org>
CRs-Fixed: 637365
Merge "[media] v4l2-ioctl.c: handle PREPARE_BUF like QUERYBUF"