i915: Fix GR register array size off-by-one bug

Make sure we have enough room for all the GR registers or we'll end up
clobbering the AR index register (which should actually be harmless
unless the BIOS is making an assumption about it).

Noticed-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>