drm/virtio: Fix NULL dereference error in virtio_gpu_poll

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 <chzigotzky@xenosoft.de>
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20211104214249.1802789-1-vivek.kasireddy@intel.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>