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