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