drm/amd/display: cap DCFCLK hardmin to 507 for NV10

[why]
Due to limitation in SMU/PPLIB, it is not possible to know Fmax @ Vmin for DCFCLK.
This causes issues at high display configurations where extra headroom of DCFCLK
can enable P-state switching

[how]
Use existing override logic.  If override not defined, then force
min = 507

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>