doc-rst: dmabuf: Fix the cross-reference

Fixes this warning:
    Documentation/linux_tv/media/v4l/dmabuf.rst:150: WARNING: undefined label: vidioc_dqbuf (if the link has no caption the label must precede a section header)

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