Fix the use case of plane_block_idx in sub8x8 RD

This commit fixes the use case of plane_block_idx, which determines
the plane (Y/U/V) index based on block index. When block idx >= 4 in
sub8x8 block loop, it should be of chroma components.

Change-Id: I072705aa7b35445524ac607089ca8ce54b7ba478