Fix PICK_MODE_CONTEXT index in non-RD coding mode

This commit fixes a bug in the PICK_MODE_CONTEXT index for
horizontal partition case. The compression performance change
is less than 0.01% level, since most blocks are selected to
use square block size in RTC coding mode.

Change-Id: I67effc18ae8795fccdd82a55f4efc609fa5cb3e1
Merge "Fix PICK_MODE_CONTEXT index in non-RD coding mode"