drm/vmwgfx: switch to ttm_sg_tt_init

According to Daniel VMWGFX doesn't support DMA-buf anyway.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/403834/