media: hantro: Avoid redundant hantro_get_{dst,src}_buf() calls

Getting the next src/dst buffer is relatively expensive
so avoid doing it multiple times.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>