msm: vidc: cleanup vidc function to register o/p buf

This change modularizes map_and_register_buf function
used to add o/p buffer to registered buffer list.

Change-Id: I73c1d4b5c66f75fe4b1bce2c2bfc2277a6157e0a
Signed-off-by: Arun Menon <avmenon@codeaurora.org>