drm/i915: Sort the shadow register table

Also verify the order at runtime. This was we can start using
binary search on it in a following patch.

v2: Add comment on the sorted array and only check it when
    debug option is enabled.

v3: Use IS_ENABLED. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> (v1)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>