V4L2 specifies different controls for the QP values of different codecs.

Simplify users that just care for the QP values by providing accessor
function that return the correct control based on the currently set
codec.
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>