drm/i915: Warn if there's a cdclk change in progess

If someone is interested in the current cdclk frquency it should
be stable and not in process of changing frquency. Warn if the current
and requested cdclk don't match in .get_display_clock_spee() on vlv.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -5275,6 +5275,10 @@ static int valleyview_get_display_clock_speed(struct drm_device *dev)
 
 	divider = val & DISPLAY_FREQUENCY_VALUES;
 
+	WARN((val & DISPLAY_FREQUENCY_STATUS) !=
+	     (divider << DISPLAY_FREQUENCY_STATUS_SHIFT),
+	     "cdclk change in progress\n");
+
 	return DIV_ROUND_CLOSEST(vco << 1, divider + 1);
 }