drm/ttm: fix, avoid iomapping system memory

If the memory is not iomem we should not try to
ioremap it. Should fix :

https://bugs.freedesktop.org/show_bug.cgi?id=27822



Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Tested-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>