media: hantro: Simplify buffer helpers

Modify hantro_get_ref() and hantro_h264_get_ref_buf() helpers
to return the buffer DMA address, this makes the code simpler
and at the same time will allow easier enablement of the
post-processing feature.

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