media: allegro: implement scaling of cpb size in SPS

The cbp_size_value_minus1 in the SPS can scaled using cpb_size_scale to
reduce the number of bits necessary to encode the value. For simplicity,
the scaling was set to 1.

Restructure to the code to make it easier to drop the cbp_size from the
channel and as we are at it, also properly implement the scaling.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>