drm/i915/skl: Fail the flip if no FB for WM calculation

Fail the flip if no FB is present but plane_state is set as visible.
Above is not a valid combination so instead of continue fail the flip.
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517115831.13830-6-mahesh1.kumar@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -4408,7 +4408,8 @@ skl_compute_wm_level(const struct drm_i915_private *dev_priv,
 	if (!intel_pstate)
 		intel_pstate = to_intel_plane_state(plane->state);
 
-	WARN_ON(!intel_pstate->base.fb);
+	if (WARN_ON(!intel_pstate->base.fb))
+		return -EINVAL;
 
 	ddb_blocks = skl_ddb_entry_size(&ddb->plane[pipe][intel_plane->id]);