fix CONFIG_HIGHMEM compile error in drivers/gpu/drm/i915/i915_gem.c

commit 9b7530cc

 ("i915: cleanup coding
horrors in i915_gem_gtt_pwrite()")

broke the i386 build for CONFIG_HIGHMEM=y.

Caught by automatic testing http://www.tglx.de/autoqa-logs/000137-0006-0001.log

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[ My bad. It's the same patch I sent out earlier, nobody noticed then either.. ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>