media: media videobuf2: Stop direct calls to queue num_buffers field

Use vb2_get_num_buffers() to avoid using queue num_buffers field directly.
This allows us to change how the number of buffers is computed in the
future.

Fixes: d055a76c ("media: core: Report the maximum possible number of buffers for the queue")
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>