VP9: Eliminate unnecessary nearest/near searches

Prior to this patch, read_inter_block_mode_info() would
find the nearmv and nearestmv for all modes.  Now it does not
search for ZEROMV modes and breaks out early for NEARMV and
NEWMV modes.

Change-Id: Ifa7b1eaf58bb03b9c7792ea5012fef477527d0fd
4 files changed