i965: Allocate register sets at screen creation, not context creation.

i965: Allocate register sets at screen creation, not context creation.

Register sets depend on the particular hardware generation, but don't
depend on anything in the actual OpenGL context.  Computing them is
fairly expensive, and they take up a large amount of memory.  Putting
them in the screen allows us to compute/allocate them once for all
contexts, saving both time and space.

Improves the performance of a context creation/destruction
microbenchmark by about 3x on my Haswell i7-4750HQ.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>