drm/vmwgfx: Fix oops on failing bo pin

When bo pin failed during modesetting,
vmwgfx would try to unref a non-existing buffer object.

Signed-off-by: Thomas Hellstrom <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

···
 			       &vmw_vram_ne_placement,
 			       false, &vmw_dmabuf_bo_free);
 	vmw_overlay_resume_all(dev_priv);
+	if (unlikely(ret != 0))
+		vfbs->buffer = NULL;
 
 	return ret;
 }
···
 	struct ttm_buffer_object *bo;
 	struct vmw_framebuffer_surface *vfbs =
 		vmw_framebuffer_to_vfbs(&vfb->base);
+
+	if (unlikely(vfbs->buffer == NULL))
+		return 0;
 
 	bo = &vfbs->buffer->base;
 	ttm_bo_unref(&bo);