drm/i915: Assert that HSW/BDW LCPLL is using the non-SSC reference

Only the non-SSC reference is truly supported for the LCPLL. Assert
that it is indeed selected.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604200933.29417-4-ville.syrjala@linux.intel.comReviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_display_power.c

@@ -3665,6 +3665,9 @@ static void hsw_assert_cdclk(struct drm_i915_private *dev_priv)
 
 	if (val & LCPLL_PLL_DISABLE)
 		DRM_ERROR("LCPLL is disabled\n");
+
+	if ((val & LCPLL_REF_MASK) != LCPLL_REF_NON_SSC)
+		DRM_ERROR("LCPLL not using non-SSC reference\n");
 }
 
 static void assert_can_disable_lcpll(struct drm_i915_private *dev_priv)