Eliminated prev_mip memsets/memcpys

For 1080 material, this buffer is currently 2,270,928 bytes.  This patch swaps
ptrs instead of copying and uses the last show_frame flag instead of setting
the entire buffer to zero.  For the test clip used, the decoder improved by up
to 1%.


Change-Id: I686825712ad56043e09ada9808dc489f875a6ce0