Speed 6 uses small tx size, namely 8x8. max_intra_bsize needs to
be modified accordingly to ensure valid intra mode checking.
Borg test on RTC set showed an overall PSNR gain of 0.335% in speed
-6.

This also changes speed -5 encoding by allowing DC_PRED checking
for block32x32. Borg test on RTC set showed a slight PSNR gain of
0.145%, and no noticeable speed change.

Change-Id: I1502978d8fbe265b3bb235db0f9c35ba0703cd45