UPSTREAM: 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>
(cherry picked from commit d89c0c83)

BUG=b:176158390, b:203009046, b:230100768
TEST=gfxbench and deqp-cts in arcvm

Signed-off-by: Rob Clark <robdclark@chromium.org>
Change-Id: Ia3f0fa03cb2a32d20cc19c7875b7c18be2b49758
Disallow-Recycled-Builds: test-failures
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3602853


Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Chia-I Wu <olv@google.com>
Reviewed-by: Ryan Neph <ryanneph@google.com>