media: sti: Use vb2_get_buffer

Use the newly introduced vb2_get_buffer API and avoid
accessing buffers in the queue directly.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>