drm/virtio: use drm_poll(..) instead of virtio_gpu_poll(..)

drm/virtio: use drm_poll(..) instead of virtio_gpu_poll(..)

With the use of dummy events, we can drop virtgpu specific
behavior.

Fixes: cd7f5ca33585 ("drm/virtio: implement context init: add virtio_gpu_fence_event")
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/20211122232210.602-3-gurchetansingh@google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>