drm/virtio: add in/out fence support for explicit synchronization

When the execbuf call receives an in-fence it will get the dma_fence
related to that fence fd and wait on it before submitting the draw call.

On the out-fence side we get fence returned by the submitted draw call
and attach it to a sync_file and send the sync_file fd to userspace. On
error -1 is returned to userspace.

VIRTGPU_EXECBUF_FENCE_FD_IN & VIRTGPU_EXECBUF_FENCE_FD_OUT
are supported at the simultaneously and can be flagged
for simultaneously.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181112165157.32765-4-robert.foss@collabora.com
Suggested-by: Rob Herring <robh@kernel.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>