drm/virtio: implement context init: add virtio_gpu_fence_event

drm/virtio: implement context init: add virtio_gpu_fence_event

Similar to DRM_VMW_EVENT_FENCE_SIGNALED.  Sends a pollable event
to the DRM file descriptor when a fence on a specific ring is
signaled.

One difference is the event is not exposed via the UAPI -- this is
because host responses are on a shared memory buffer of type
BLOB_MEM_GUEST [this is the common way to receive responses with
virtgpu].  As such, there is no context specific read(..)
implementation either -- just a poll(..) implementation.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Acked-by: Nicholas Verne <nverne@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20210921232024.817-12-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>