drm/i915/display: Warn if the FBC is still writing to stolen on removal

If the FBC is still writing into stolen, it will overwrite any future
users of that stolen region. Check before release, just to ease any
concerns -- we can remove it again later if it is barking up the wrong
tree.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/1635
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200503180034.20010-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/display/intel_fbc.c

@@ -540,6 +540,9 @@ static void __intel_fbc_cleanup_cfb(struct drm_i915_private *dev_priv)
 {
 	struct intel_fbc *fbc = &dev_priv->fbc;
 
+	if (WARN_ON(intel_fbc_hw_is_active(dev_priv)))
+		return;
+
 	if (!drm_mm_node_allocated(&fbc->compressed_fb))
 		return;