[media] DocBook: add dmabuf as streaming I/O in VIDIOC_REQBUFS description

[media] DocBook: add dmabuf as streaming I/O in VIDIOC_REQBUFS description

Commit df8a908a2f7fb ("[media] Documentation: media: description of DMABUF
importing in V4L2") documented the V4L2 dma-buf importing support but did
not update the VIDIOC_REQBUFS description, so only the Memory Mapping and
User Pointer I/O methods are mentioned.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>