Drivers like vmwgfx may want to test whether the dma page pool is present
or not. Since it's activated by default by TTM if compiled-in, define a
hidden configuration option that the driver can test for.

Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>