drm/ttm: add transparent huge page support for DMA allocations v2

Try to allocate huge pages when it makes sense.

v2: fix comment and use ifdef

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>