drm: Use ioremap_wc in i915_driver instead of ioremap, since we always want WC.

Fixes failure to map the ringbuffer when PAT tells us we don't get to do
uncached on something that's already mapped WC, or something along those lines.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>