i965: Implement L3 state atom.

i965: Implement L3 state atom.

The L3 state atom calculates the target L3 partition weights when the
program bound to some shader stage is modified, and in case they are
far enough from the current partitioning it makes sure that the L3
state is re-emitted.

v2: Fix for inconsistent units the context URB size is expressed in.
    Clamp URB size to 1008 KB on SKL due to FF hardware limitation.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg  <krh@bitplanet.net>