drm/amd/display: add hubbub_init related

Required by display init, also update get_dig_mode

Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Reviewed-by: Hansen Dsouza <hansen.dsouza@amd.com>
Reviewed-by: Duncan Ma <duncan.ma@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>