For no obvious reason VB2_DMABUF was not enabled. Add this.

Also both read and write file ops were set, but this was not enabled
in the vb2 io_modes or in the device_caps capabilities. Drop this.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>