Revert "drm/virtio: Fix NULL vs IS_ERR checking in virtio_gpu_object_shmem_init"

This reverts commit 72893aad which is
commit c2496873 upstream.

It is needed to be reverted so we can drop a larger ABI-breaking commit.
It will be brought back again in a later commit.

Bug: 161946584
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I2505a5474baa478bf44b0f5eb4799ba90132113c