Eliminated prev_mip memsets/memcpys in encoder

This patch swaps ptrs instead of copying and uses the
last show_frame flag instead of setting the entire buffer
to zero.  On the decode side, forced prev_mi to always point
to a valid mode info buffer.  Required by the next frame.

Change-Id: I90441eaf087868e9f9bc368e15e0becc848c4a51
Trivial change to add crude timing information for encoding section

Change-Id: I84e07c2f1240b95d5de083df06eb3d581bfb9b68