drm/i915/skl: Add warn about unsupported CDCLK rates

While checking workarounds related to the CDCLK PLL, I noticed that the
DMC firmware bits for WA#1183 are missing for SKL. After that I
clarified with HW people that it's not needed on SKL, since it doesn't
support eDP1.4 which would be the only thing requiring the problematic
CDCLK clock rates. So in theory we shouldn't ever choose these
frequencies, but add an assert in any case for catching such cases and
for documentation.

v2:
- Move the check to skl_set_cdclk and warn whenever using the
  corresponding VCO freq. (Ville)

v3:
- Actually check for the platform. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180608144137.7943-1-imre.deak@intel.com

drivers/gpu/drm/i915/intel_cdclk.c

@@ -991,6 +991,16 @@ static void skl_set_cdclk(struct drm_i915_private *dev_priv,
 	u32 freq_select, cdclk_ctl;
 	int ret;
 
+	/*
+	 * Based on WA#1183 CDCLK rates 308 and 617MHz CDCLK rates are
+	 * unsupported on SKL. In theory this should never happen since only
+	 * the eDP1.4 2.16 and 4.32Gbps rates require it, but eDP1.4 is not
+	 * supported on SKL either, see the above WA. WARN whenever trying to
+	 * use the corresponding VCO freq as that always leads to using the
+	 * minimum 308MHz CDCLK.
+	 */
+	WARN_ON_ONCE(IS_SKYLAKE(dev_priv) && vco == 8640000);
+
 	mutex_lock(&dev_priv->pcu_lock);
 	ret = skl_pcode_request(dev_priv, SKL_PCODE_CDCLK_CONTROL,
 				SKL_CDCLK_PREPARE_FOR_CHANGE,