media: allegro: extract RBSP handler from H.264 NAL generator

The RBSP structure is the same for HEVC and H.264. In order to be able
to reuse the RBSP handler for generating HEVC NAL units, extract the
functions from the H.264 generator.

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>