drm/i915: Describe structure member in documentation

Add description of wakeref member of intel_shared_dpll
structure to documentation.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Anna Karas <anna.karas@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191008092849.6511-1-anna.karas@intel.com

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

@@ -337,6 +337,11 @@ struct intel_shared_dpll {
 	 * @info: platform specific info
 	 */
 	const struct dpll_info *info;
+
+	/**
+	 * @wakeref: In some platforms a device-level runtime pm reference may
+	 * need to be grabbed to disable DC states while this DPLL is enabled
+	 */
 	intel_wakeref_t wakeref;
 };