media: venus: venc: Add support for frame-skip mode v4l2 control

This adds support for frame-skip-mode standard v4l2 control in
encoder driver. The control is implemented based on the
combination of client selected bitrate-mode and frame-skip-mode.
Once The client selected bitrate-mode (constant or variable) and
the frame-skip-mode is not disabled we set variable framerate for
rate controller.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>