drm/amd/display: Set init freq for DCFCLK DS

[Description]
- Set init frequency for DCFCLK DS
- For now choose 10Mhz after turning off all pipes on init
- DCN32 spreadsheet shows 8Mhz is min for any display config

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>