drm/i915: Use ilk_max_pixel_rate() for BXT cdclk calculation

BXT uses the "pch" panel fitter configuration, so we can use
ilk_max_pixel_rate() instead of intel_mode_max_pixclk() to compute the
pipe pixel rate. ilk_max_pixel_rate() will account for the pipe
scaler downscaling factor whereas intel_mode_max_pixclk() will not.

I'm pretty sure the same limitation is there on GMCH platforms, but
no one just bothered to implement the downscaling adjustment for them.
Probably should just unify the panel fitter setup more across the
platforms and use the exact same code on all platforms for this.
But in the meantime, let's at least make BXT a bit more correct.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462995892-32416-6-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -117,6 +117,7 @@ static void ironlake_pfit_disable(struct intel_crtc *crtc, bool force);
 static void ironlake_pfit_enable(struct intel_crtc *crtc);
 static void intel_modeset_setup_hw_state(struct drm_device *dev);
 static void intel_pre_disable_primary_noatomic(struct drm_crtc *crtc);
+static int ilk_max_pixel_rate(struct drm_atomic_state *state);
 
 struct intel_limit {
 	struct {
@@ -5963,8 +5964,7 @@ static int valleyview_modeset_calc_cdclk(struct drm_atomic_state *state)
 
 static int broxton_modeset_calc_cdclk(struct drm_atomic_state *state)
 {
-	struct drm_device *dev = state->dev;
-	int max_pixclk = intel_mode_max_pixclk(dev, state);
+	int max_pixclk = ilk_max_pixel_rate(state);
 	struct intel_atomic_state *intel_state =
 		to_intel_atomic_state(state);