msm: vidc: Allow decoder clients to query entropy

Implement READ_ONLY support for V4L2_CID_MPEG_VIDEO_H264_ENTROPY_MODE so
that clients can check the coding type of the bitstream that's attempted
to be decoded.

Also unlock the controls so that leaf level functions can call g_ctrl.

Change-Id: Id51c99a220cee73bd3e1e77bba92b5804c9c6de7
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>