drm/amd/display: update dcn21 hubbub registers

use dcn20 common regs define to share some regs with dcn20

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>