i965/vec4: Calculate register allocation q values manually.

i965/vec4: Calculate register allocation q values manually.

This fixes a regression in the running time of Piglit introduced by
commit 78e9043475d4bed8b50f7e413963c960fa0935bb, which increased the
number of register allocation classes set up by the VEC4 back-end
from 2 to 16.  The algorithm used by ra_set_finalize() to calculate
them is unnecessarily expensive, do it manually like the FS back-end
does.

Reported-by: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>