media: hantro: Use post processor scaling capacities

Hantro G2 post processor is able to down scale decoded frames
by a factor of 2, 4 or 8.
Add enum_framesizes() ops to postproc_ops structure to enumerate the
possible output sizes for a given input resolution.
For G2 post-processor use fsize->index (from 0 to 3) as power of 2
divisor. As described in v4l2 documentation return -EINVAL when scaling
down isn't possible.

fluster scores:
77/147 for HEVC
143/303 for VP9

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>