vp9_reconintra: simplify d45_predictor

only the immediate above right pixel is needed; 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_right

Change-Id: Ica4f2b8d25eec3ca1d6fa52ef0d4adc228eeea3f