Encoding: Add one ROI flag and ROI buffer

Encoding: Add one ROI flag and ROI buffer

v3:
free the qp_per_mb for vme context

v2:
remove unused variable to avoid warning when compiling.

v1:
Add one flag to indicate whether ROI is supported in one encode context
Allocate one ROI buffer to hold qp per mb dynamically

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: pjl <cecilia.peng@intel.com>
Signed-off-by: Pengfei Qu <Pengfei.Qu@intel.com>