drm/i915: Store the active forcewake range table pointer

If we store this in the uncore structure we are on a good way to
show more commonality between the per-platform implementations.

v2: Constify table pointer and correct coding style. (Chris Wilson)
v3: Rebase.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>