drm/i915: Restore the invalid access without RPM warning

A long time ago we turned off the warning as it was too painful, we had
too much broken code. Turn it back on now as we are mostly clean and
need to prevent returning to such orangeness.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302074157.21631-2-chris@chris-wilson.co.uk
Reviewed-by: Imre Deak <imre.deak@intel.com>