Fix rd_pick_partition search loop for 4x4 blocks

The partition search for 4x4 blocks takes unnecessary steps to
reconstruct pixels and an extra partition type update. This commit
removes such operations. No visible compression/speed difference.
Thanks to Yue (yuec@) for finding this issue.

Change-Id: I3f83824aa3fd3717d63be0b280fa57258939a70a