i965: Allocate push constant L3 space once at startup on Gen7+.

i965: Allocate push constant L3 space once at startup on Gen7+.

We always allocate the maximum amount of space and never change it, so
it makes sense to do it once.  Programming it on startup also lets us
skip re-programming it from BLORP.

This removes a tiny amount of overhead from our drawing loop.

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