drm/i915: Data driven register to forcewake domains lookup

Move finding the correct forcewake domains to take for
register access from code to a mapping table. This will
allow more interesting work in the following patches
and is easier to review if singled out early.

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