drm/amd/display: Fix 6x4K displays light-up on Vega20 (v2)

[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.

v2: backport to 4.20 (Alex)
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>

drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c

@@ -2512,6 +2512,8 @@ static void pplib_apply_display_requirements(
 			dc,
 			context->bw.dce.sclk_khz);
 
+	pp_display_cfg->min_dcfclock_khz = pp_display_cfg->min_engine_clock_khz;
+
 	pp_display_cfg->min_engine_clock_deep_sleep_khz
 			= context->bw.dce.sclk_deep_sleep_khz;