media: i2c: video-i2c: Stop direct calls to queue num_buffers field

media: i2c: video-i2c: 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.
Fix the number of buffers computation at the same time.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
CC: Matt Ranostay <matt.ranostay@konsulko.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>