media: hantro: Add hantro_get_{src, dst}_buf() helpers

And replace all calls to v4l2_m2m_next_{src,dst}_buf() by
hantro_get_{src,dst}_buf() one.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
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>