This add the support for the new fault callback, does change anything
from driver point of view, thought it should allow nouveau to add
support for unmappable VRAM.

Improvement: store the aperture base in a variable so that we don't
call a function to get it on each fault.

Patch hasn't been tested on any hw.

V2 don't derefence bo->mem.mm_node as it's not NULL only for
   VRAM or GTT
V3 update after io_mem_reserve/io_mem_free callback balancing
V4 callback has to ioremap
V5 ioremap is done by ttm
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>