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: I37efa1e63f9927c17658a1fee8dfe48046d3a4e8
Signed-off-by: Jia Meng <jmeng@codeaurora.org>