drm/ttm: Fix ttm in-kernel copying of pages with non-standard caching attributes.

For x86 this affected highmem pages only, since they were always kmapped
cache-coherent, and this is fixed using kmap_atomic_prot().

For other architectures that may not modify the linear kernel map we
resort to vmap() for now, since kmap_atomic_prot() generally uses the
linear kernel map for lowmem pages. This of course comes with a
performance impact and should be optimized when possible.

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