Allow large tx_size in lossless coding with transform skipping

In lossless coding, tx_size can be larger than 4x4 when transform
skipping is activated. Compared to regular vp9 lossless coding,
performance improvement for derf is about 5%; gain is larger
for screen content videos.

Change-Id: Ib20ece7e117f29fb91543612757302a2400110b4
Merge "Allow large tx_size in lossless coding with transform skipping" into nextgen