mm-video: vidc: fix memory leak in dynamic buffer mode

In dynamic buffer mode, for non-secure playback,
output buffer is mapped before each ftb to driver.
This buffer is unmapped in the fbd. During flush
operations, the mapped buffer in ftb is not freed
leading to memory leak. To fix the issue, move
the memory mapping to just before sending the ftb
to driver.

Bug: 11973141

Change-Id: Ib0e5525fdf8132434800bf4d4ff8d27b085fc8fc