drm/i915/dg1: DG1 does not support DC6

DC6 is not supported on DG1, so change the allowed DC mask for DG1.
This is not yet on bspec, but it has been confirmed by HW engineers.

Cc: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201014191937.1266226-9-lucas.demarchi@intel.com

drivers/gpu/drm/i915/display/intel_display_power.c

@@ -4492,7 +4492,10 @@ static u32 get_allowed_dc_mask(const struct drm_i915_private *dev_priv,
 	int max_dc;
 
 	if (INTEL_GEN(dev_priv) >= 12) {
-		max_dc = 4;
+		if (IS_DG1(dev_priv))
+			max_dc = 3;
+		else
+			max_dc = 4;
 		/*
 		 * DC9 has a separate HW flow from the rest of the DC states,
 		 * not depending on the DMC firmware. It's needed by system