drm/qxl: use ttm_tt

qxl device will not dma, so we don't need ttm_dma_tt.  Go use ttm_tt
instead, to avoid wasting resources (swiotlb bounce buffers for
example).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190129082541.1392-1-kraxel@redhat.com