DO NOT MERGE: mm-video-v4l2: venc: Disallow changing buffer count/size on allocated port

Count and size negotiation of port-buffers should only be allowed
when the port hasn't been allocated yet.
Letting the client change count/size on a pre-allocated port will
cause inconsistencies in the count/size of memory allocated for
headers and internal lists.
Fix resetting of buffer-base (m_inp_mem_ptr) when all buffers are freed,
for all the buffer-modes.

Bug: 29421682
Fixes: Local Privilege Escalation in MediaServer (libOmxVenc problem #10)

Change-Id: I9abead969bc3c908e6db9beb6316fd572dac25f7