Set scale factors consistently for SPLITMV

This commit updates the 4x4 prediction to consistently use the
build_2x1_inter_predictor() method. That function is updated to
calculate the scale offset, rather than relying on the caller
to calculate it. In the case that the 2x1 prediction can not
be used, the scale offset is recalculated for each 1x1 block.
The idea here is that the offsets are calculated before each
call to vp9_build_scaled_inter_predictor().

Change-Id: I0ac3343dd54e2846efa3c4195fcd328b709ca04d