drm/i915: All fw_domains share the same set/clear/reset values

Since we reuse the same values for each fw_domain, move them onto
uncore.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323101944.21627-6-chris@chris-wilson.co.uk