media: allegro: make QP configurable

The V4L2_CID_MPEG_VIDEO_FRAME_RC_ENABLE control allows to enable/disable
rate control on a channel. When rate control is disabled, the driver
shall use constant QP, which are set by the application. Also implement
the controls for configuring the QP.

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>