drm: Use pgprot_writecombine in GEM GTT mapping to get the right bits for !PAT.

Otherwise, the PAGE_CACHE_WC would end up getting us a UC-only mapping, and
the write performance of GTT maps dropped 10x.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[anholt: cleaned up unused var]
Signed-off-by: Eric Anholt <eric@anholt.net>