media: coda: add RC enable controls

Currently the encoder enables the rate control algorithms if the bitrate
control is non-zero. Implement the V4L2_CID_MPEG_VIDEO_FRAME_RC_ENABLE
and V4L2_CID_MPEG_VIDEO_MB_RC_ENABLE controls to allow userspace to
choose frame-level or macroblock-level rate control updates, or to
explicitly disable rate control. Both controls are initially enabled to
keep the current behavior.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>