drm/amd/display: Query VCO frequency from register for DCN3.1

drm/amd/display: Query VCO frequency from register for DCN3.1

[Why]
Hardcoding the VCO frequency isn't correct since we don't own or control
the value.

In the case where the hardcode is also missing we can't lightup display.

[How]
Query from the CLK register instead. Update the DFS frequency to be able
to compute the VCO frequency.

Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>