[media] doc-rst: add dmabuf as streaming I/O in VIDIOC_REQBUFS description

Commit 707e65831d3b("[media] DocBook: add dmabuf as streaming I/O
in VIDIOC_REQBUFS description") added DMABUF to reqbufs description,
but, as we're migrating to ReST markup, we need to keep it in sync
with the change.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>