i965 gen6: Allocate URB space for GS

i965 gen6: Allocate URB space for GS

When the GS is not in use, the entire URB space is available for the
VS.  When the GS is in use, we split the URB space 50/50.

The 50/50 split is probably not optimal--we'll probably want tune this
for performance in a future patch.  For example, in most situations,
it's probably worth allocating more than 50% of the space to the VS,
since VS space is used for vertex caching.  But for now this is good
enough.

Based on previous work by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>