drm/i915/tgl: Add DC3CO mask to allowed_dc_mask and gen9_dc_mask

Enable dc3co state in enable_dc module param and add dc3co
enable mask to allowed_dc_mask and gen9_dc_mask.

v1: Adding enable_dc=3,4 options to enable DC3CO with DC5 and DC6
    independently. [Animesh]
v2: Using a switch statement for cleaner code. [Animesh]

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191003081738.22101-3-anshuman.gupta@intel.com