msm: vidc: Fix possible UAF during buffer unregister call

[1] During buffer unregister, CVP buffer lock is released immediately
after finding buffer in register buffer list. UAF might happen if two
threads execute same unregister command as buffer free happens after
unregister done.
[2] Hold CVP buffer lock through out in unregister and unregister done
calls.

Change-Id: I8b6734410369ab990081c558ba846b6dfbfc8588
Signed-off-by: Vasantha Balla <quic_vballa@quicinc.com>