drm/ttm: add ttm_sg_tt_init

This allows drivers to only allocate dma addresses, but not a page
array.

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