drm/virtio: Fix NULL dereference error in virtio_gpu_poll

When virgl is not enabled, vfpriv pointer would not be allocated.
Therefore, check for a valid value before dereferencing.

Reported-by: Christian Zigotzky <[e-mail]>
Cc: Gurchetan Singh <[e-mail]>
Cc: Gerd Hoffmann <[e-mail]>
Signed-off-by: Vivek Kasireddy <[e-mail]>
Tested-by: Christian Zigotzky <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Gerd Hoffmann <[e-mail]>

drivers/gpu/drm/virtio/virtgpu_drv.c

···
 	struct drm_file *drm_file = filp->private_data;
 	struct virtio_gpu_fpriv *vfpriv = drm_file->driver_priv;
 	struct drm_device *dev = drm_file->minor->dev;
+	struct virtio_gpu_device *vgdev = dev->dev_private;
 	struct drm_pending_event *e = NULL;
 	__poll_t mask = 0;
 
-	if (!vfpriv->ring_idx_mask)
+	if (!vgdev->has_virgl_3d || !vfpriv || !vfpriv->ring_idx_mask)
 		return drm_poll(filp, wait);
 
 	poll_wait(filp, &drm_file->event_wait, wait);