Moved use_prev_in_find_mv_refs check to frame level

This patch checks at the frame level to see if the previous
mode info context can be used.  This patch eliminates the
flag check that was done for every mode and removes another
check that was done prior to every vp9_find_mv_refs().

Change-Id: I9da5e18b7e7e28f8b1f90d527cad087073df2d73