media: hantro: Prepare for other G2 codecs

VeriSilicon Hantro G2 core supports other codecs besides hevc.
Factor out some common code in preparation for vp9 support.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>