DO NOT MERGE ANYWHERE: mm-video-v4l2: venc: Disallow changing buffer

count/size on allocated port

[a re-packaging of the LMP version directly from qcom so that it applies
against the LMP-MR1 codebase; no actual code changes, just updated context
for where the changes are applied within the files]

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
Change-Id: I2bcc8f07c35488d3ea62fdcad107881349de1e2a
Fixes: Local Privilege Escalation in MediaServer (libOmxVenc problem #10)