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 <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -720,6 +720,8 @@ static int vmw_surface_dmabuf_pin(struct vmw_framebuffer *vfb)
 			       &vmw_vram_ne_placement,
 			       false, &vmw_dmabuf_bo_free);
 	vmw_overlay_resume_all(dev_priv);
+	if (unlikely(ret != 0))
+		vfbs->buffer = NULL;
 
 	return ret;
 }
@@ -730,6 +732,9 @@ static int vmw_surface_dmabuf_unpin(struct vmw_framebuffer *vfb)
 	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);
 	vfbs->buffer = NULL;