drm/ttm: Fix cached TTM page allocation.

This patch fixes a regression introduced with the pool page allocator
in the event that there are no highmem pages (for example x86_64),
in which case cached page allocation would fail.

Tested with the vmwgfx driver on a 64-bit vm.

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