drm/amd/display: Fix 6x4K displays light-up on Vega20

[Why]
More than 4x4K didn't lightup on Vega20 due to low dcfclk value.
Powerplay expects valid min requirement for dcfclk from DC.

[How]
Update min_dcfclock_khz based on min_engine_clock value.

Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>