drm/i915: Sort forcewake mapping tables

Sorting the tables (verified at runtime to help during
development) is another prerequisite for interesting
work which will follow.

v2:
 * Remove const away cast and improve comments. (Chris Wilson)
 * Check tables only when debug option is enabled.

v3: Use IS_ENABLED. (Chris Wilson)

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