Separate encode_block for pass 1 and 2

The encode_block for pass 1 takes simpler functionalities and can
save a few branches. The main reason is to make encode_block only
used after running rate-distortion optimization search in pass 2,
hence allowing dual buffer stack approach later.

Change-Id: I9e549ffb758e554fe185e48a07d6e0e01e475bcf