drm/virtio: add uapi for in and out explicit fences

Add a new field called fence_fd that will be used by userspace to send
in-fences to the kernel and receive out-fences created by the kernel.

This uapi enables virtio to take advantage of explicit synchronization of
dma-bufs.

There are two new flags:

* VIRTGPU_EXECBUF_FENCE_FD_IN to be used when passing an in-fence fd.
* VIRTGPU_EXECBUF_FENCE_FD_OUT to be used when requesting an out-fence fd

The execbuffer IOCTL is now read-write to allow the userspace to read the
out-fence.

On error -1 should be returned in the fence_fd field.

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/20181112165157.32765-3-robert.foss@collabora.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>