drm/i915/tgl: Add DC3CO counter in i915_dmc_info

Adding DC3CO counter in i915_dmc_info debugfs will be
useful for DC3CO validation.
DMC firmware uses DMC_DEBUG3 register as DC3CO counter
register on TGL, as per B.Specs DMC_DEBUG3 is general
purpose register.

v1: comment modification for DMC_DBUG3.
    using GEN >= 12 check instead of IS_TIGERLAKE()
    to print DMC_DEBUG3 counter value.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: 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/20191003081738.22101-7-anshuman.gupta@intel.com