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