x86: Export kmap_atomic_prot() needed for TTM.

This functionality is needed to kmap_atomic() highmem pages that may
potentially have or are about to set up other mappings with
non-standard caching attributes.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>