msm: vidc: acquire lock for complete register sequence

Register pkt is queued to firmware, and it is processed
and register_done is getting called from reverse thread
before buffer is added to list in forward thread. To
avoid this race issue, acquired the cvpbuf.lock before
posting register command to firmware.

Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>