msm: vidc: Reject read only OPB buffer in split mode

Requirement stipulates OPB to be writable buffer in split mode.
Erring out in case of read only buffer will help debugging quickly.

Change-Id: I529a97b3d0aa5f82e7cf7d024475338a9d24d1dd
Signed-off-by: Amit Shekhar <ashekhar@codeaurora.org>