drm/ttm, drm/vmwgfx: Use a configuration option for the TTM dma page pool

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>