drm/i915: Fix documentation for intel_dpll_get_freq()

Fix the following kerneldoc warning and while at it also the doc for the
corresponding vfunc hook.

$ make htmldocs 2>&1 > /dev/null | grep i915
./drivers/gpu/drm/i915/display/intel_dpll_mgr.h:285: warning: Function parameter or member 'get_freq' not described in 'intel_shared_dpll_funcs'
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304150918.25473-1-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_dpll_mgr.c

@@ -4408,6 +4408,13 @@ void intel_update_active_dpll(struct intel_atomic_state *state,
 	dpll_mgr->update_active_dpll(state, crtc, encoder);
 }
 
+/**
+ * intel_dpll_get_freq - calculate the DPLL's output frequency
+ * @i915: i915 device
+ * @pll: DPLL for which to calculate the output frequency
+ *
+ * Return the output frequency corresponding to @pll's current state.
+ */
 int intel_dpll_get_freq(struct drm_i915_private *i915,
 			const struct intel_shared_dpll *pll)
 {

drivers/gpu/drm/i915/display/intel_dpll_mgr.h

@@ -279,6 +279,12 @@ struct intel_shared_dpll_funcs {
 			     struct intel_shared_dpll *pll,
 			     struct intel_dpll_hw_state *hw_state);
 
+	/**
+	 * @get_freq:
+	 *
+	 * Hook for calculating the pll's output frequency based on its
+	 * current state.
+	 */
 	int (*get_freq)(struct drm_i915_private *i915,
 			const struct intel_shared_dpll *pll);
 };