drm/amd/display: fix dccg root clock optimization related hang

[Why]
enable dpp rcg before we disable dppclk in hw_init cause system
hang/reboot

[How]
we remove dccg rcg related code from init into a separate function and
call it after we init pipe

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Qili Lu <qili.lu@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>