drm/amd/display: Adding prototype for dccg21_update_dpp_dto()

Kernel test robot throws below warning ->

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn21/dcn21_dccg.c:46:6:
warning: no previous prototype for 'dccg21_update_dpp_dto'
[-Wmissing-prototypes]

Adding prototype for dccg21_update_dpp_dto().
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn21/dcn21_dccg.h

@@ -32,5 +32,6 @@ struct dccg *dccg21_create(
 	const struct dccg_shift *dccg_shift,
 	const struct dccg_mask *dccg_mask);
 
+void dccg21_update_dpp_dto(struct dccg *dccg, int dpp_inst, int req_dppclk);
 
 #endif /* __DCN21_DCCG_H__ */