drm/i915: Document that the pll->mode_set hook is optional

The WRPLLs won't use them.
Reviewed-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -206,6 +206,8 @@ struct intel_shared_dpll {
 	/* should match the index in the dev_priv->shared_dplls array */
 	enum intel_dpll_id id;
 	struct intel_dpll_hw_state hw_state;
+	/* The mode_set hook is optional and should be used together with the
+	 * intel_prepare_shared_dpll function. */
 	void (*mode_set)(struct drm_i915_private *dev_priv,
 			 struct intel_shared_dpll *pll);
 	void (*enable)(struct drm_i915_private *dev_priv,