drm/amd/display: rename dccg to clk_mgr

In preparation for adding the actual dccg block since the
current implementation of dccg is mor eof a clock manager
than a hw block

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>