drm/virtio: Fix NULL vs IS_ERR checking in virtio_gpu_object_shmem_init

Since drm_prime_pages_to_sg() function return error pointers.
The drm_gem_shmem_get_sg_table() function returns error pointers too.
Using IS_ERR() to check the return value to fix this.

Fixes: 2f2aa137

 ("drm/virtio: move virtio_gpu_mem_entry initialization to new function")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20220602104223.54527-1-linmq006@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>