drm/virtio: add dma sync for dma mapped virtio gpu framebuffer pages

With virtio gpu ttm-pages being dma mapped, dma sync is needed when
swiotlb is used as bounce buffers, before TRANSFER_TO_HOST_2D/3D
commands are sent.

Signed-off-by: Jiandi An <jiandi.an@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180919070931.91168-1-jiandi.an@amd.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>