VT: Add OMX_Video_ControlRateConstant feature

The OMX_Video_ControlRateConstant is a sutiable OMX option
to stablize encoder bandwidth.

This helps encoder to make more regular bps.

Merged-in: I67a08be8b8a19b573c873e96d317e16c7105084e
Change-Id: I67a08be8b8a19b573c873e96d317e16c7105084e
Signed-off-by: Kim Sungyeon <sy85.kim@samsung.com>