media: vicodec: prepare support for various number of planes

Add fields to the structs `fwht_raw_frame`, `v4l2_fwht_pixfmts`
to support various number of planes - formats
with alpha channel that have 4 planes and greyscale formats
that have 1 plane.

Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>