vp9_pickmode: revert rd modeling change for hbd

Avoids a segfault in high-bitdepth builds.
This restores the condition to its state prior to:
7991241 vp9: Change the scheme for modeling rd for bsize 32x32.

BUG=webm:1250

Change-Id: I6183d5b34cb89dfbf27b7bb589812148a72cd7de
1 file changed