drm/virtgpu api: define a dummy fence signaled event

The current virtgpu implementation of poll(..) drops events
when VIRTGPU_CONTEXT_PARAM_POLL_RINGS_MASK is enabled (otherwise
it's like a normal DRM driver).

This is because paravirtualized userspaces receives responses in a
buffer of type BLOB_MEM_GUEST, not by read(..).

To be in line with other DRM drivers and avoid specialized behavior,
it is possible to define a dummy event for virtgpu.  Paravirtualized
userspace will now have to call read(..) on the DRM fd to receive the
dummy event.

Fixes: b10790434c ("drm/virtgpu api: create context init feature")
Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20211122232210.602-2-gurchetansingh@google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_drv.h

@@ -138,7 +138,6 @@ struct virtio_gpu_fence_driver {
 	spinlock_t       lock;
 };
 
-#define VIRTGPU_EVENT_FENCE_SIGNALED_INTERNAL 0x10000000
 struct virtio_gpu_fence_event {
 	struct drm_pending_event base;
 	struct drm_event event;