msm: vidc: added v4l2 control to set the color of concealed MBs.

The conceal color is set to be green by default, while some users
required it to be black.
Set it to be configurable by adding this v4l2 control.

Change-Id: I25565ee837c011f5875ce91bde23a169fbdb0709
Signed-off-by: Jia Meng <jmeng@codeaurora.org>

Conflicts:
	drivers/media/platform/msm/vidc/msm_vdec.c