mm-video: vidc: Set the output buffer size properly

The output buffer size was considered to be what the driver advertised
to the component.  However, in use buffer mode, the client might have
allocated a bigger size.  To take advantage of the larger buffer, use
the client's advertised buffer size.

CRs-Fixed: 647383
Change-Id: I1778f1767280ecbc6e128eea895e34564ef3a015