drm/amd/display: correctly populate dpp refclk in fpga

[Why]
In diags environment we are not programming the DPP DTO
correctly.

[How]
Populate the dpp refclk in dccg so it can be used to correctly
program DPP DTO.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>