[media] V4L: vb2: add support for buffers of different sizes on a single queue

The two recently added ioctl()s VIDIOC_CREATE_BUFS and VIDIOC_PREPARE_BUF
allow user-space applications to allocate video buffers of different
sizes and hand them over to the driver for fast switching between
different frame formats. This patch adds support for buffers of different
sizes on the same buffer-queue to vb2.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>