drm/ttm: fix DMA32 handling in the global page pool

When we have mixed DMA32 and non DMA32 device in one system
it could otherwise happen that the DMA32 device gets pages
it can't work with.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/401317/