media: hantro: Cleanup format negotiation helpers

Format negotiation helpers, hantro_find_format()
and hantro_get_default_fmt() can be simplified,
making the code a little bit clearer.

More importantly, this change is preparation work
for the post-processor usage.

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