[media] mtk-vcodec: convert driver to use the new vb2_queue dev field

The patch dropping the vb2_dma_contig_init_ctx() and _cleanup_ctx()
functions was already applied before this driver was added. So convert
this driver as well.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>