i965/disasm: Properly decode branch_ctrl (gen8+)

i965/disasm: Properly decode branch_ctrl (gen8+)

Add support for decoding the new branch control bit. I saw two things wrong with
the existing code.

1. It didn't bother trying to decode the bit.
-  While we do not *intentionally* emit this bit today, I think it's interesting
   to see if we somehow ended up with the bit set. It may also be useful in the
   future.

2. It seemed to be the wrong bit.
-  The docs are pretty poor wrt which bit this actually occupies. To me, it
   /looks/ like it should be bit 28. I am not sure where Ken got 30 from. I
   verified it should be 28 by looking at the simulator code.

I also added the most basic support for GOTO simply so we don't need to remember
to change the function in the future.

v2:
Move the branch_ctrl check out of the if gen >= 6 check to make it more
readable. (Matt)
ENDIF doesn't have branch_ctrl (Matt + Ken)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>