media: rockchip/vpu: Use v4l2_apply_frmsize_constraints() where appropriate

Use the v4l2_apply_frmsize_constraints() helper instead of open-coding
it.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>