Refactor row and col versions of fadst16 step2 msa functions.

Only differences:
- Initial values of 'out_ptr' were different.
- Macros used to load g13, g15, g5 and g7 were different, but they were
actually equivalent.

BUG=aomedia:442

Change-Id: I58bbb97e4d9ed3bebabaaa24442021703415aaec
1 file changed