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>