media: dvb-core: Use vb2_get_buffer() instead of directly access to buffers array

Use vb2_get_buffer() instead of direct access to the vb2_queue bufs array.
This allows us to change the type of the bufs in the future.
After each call to vb2_get_buffer() we need to be sure that we get
a valid pointer so check the return value of all of them.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>