media: videobuf2: Use vb2_get_num_buffers() helper

Stop using queue num_buffers field directly, instead use
vb2_get_num_buffers().
This prepares for the future 'delete buffers' feature where there are
holes in the buffer indices.

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