media: au0828: fix VIDEO_V4L2 dependency

media: au0828: fix VIDEO_V4L2 dependency

After the move of videobuf2 into the common directory, selecting the
au0828 driver with CONFIG_V4L2 disabled started causing a link failure,
as we now attempt to build videobuf2 but it still requires v4l2:

ERROR: "v4l2_event_pending" [drivers/media/common/videobuf/videobuf2-v4l2.ko] undefined!
ERROR: "v4l2_fh_release" [drivers/media/common/videobuf/videobuf2-v4l2.ko] undefined!
ERROR: "video_devdata" [drivers/media/common/videobuf/videobuf2-v4l2.ko] undefined!
ERROR: "__tracepoint_vb2_buf_done" [drivers/media/common/videobuf/videobuf2-core.ko] undefined!
ERROR: "__tracepoint_vb2_dqbuf" [drivers/media/common/videobuf/videobuf2-core.ko] undefined!
ERROR: "v4l_vb2q_enable_media_source" [drivers/media/common/videobuf/videobuf2-core.ko] undefined!

We want to be able to build the core au0828 support without V4L2,
so this makes the 'select' conditional on V4L2, and refines the
dependencies in VIDEO_AU0828_V4L2 so it can only be enabled in
the exact conditions that have VIDEOBUF2_VMALLOC reachable.

Fixes: 03fbdb2fc2b8 ("media: move videobuf2 to drivers/media/common")
Fixes: 05439b1a3693 ("[media] media: au0828 - convert to use videobuf2")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>