Refactor vp9_diamond_search_sad function

Currently, vp9_diamond_search_sadx4() is only called when sse3 is
enabled, which is improper since sse2 optimization of sdx4df
functions are available. Changed to always use
vp9_diamond_search_sadx4().

Change-Id: I4b95d6b7a3c6c645783c373f0ba8d645ece24717