vp9_reconintra: simplify d63_predictor

calculate the averages needed for even and odd rows once; this removes a
conditional from the inner loop
the final average calculated currently relies on above[] being extended,
it could be reduced to use
above[block_size - 2] + 3 * above[block_size - 1]

Change-Id: I70f5eac8d8a2a959c7114844a95826f445c3dd4d