drm/i915: BUG_ON() when cmd/reg tables are not sorted

As suggested during review, this makes it much more obvious
when the tables are not sorted.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>