Re-design all-zero-coeff block index buffer use

Use the zcoeff_blk buffer of PICK_MODE_CONTEXT to store the indexes
of all-zero-coeff block of the current best mode. Remove the temporary
buffer best_zcoeff_blk defined in the rate-distortion optimization
loop. This improves the speed performance by about 0.5% in all speed
settings.

Change-Id: Ie3e15988ddfa581eafa2e19a8228d3fe4a46095c