drm/i915/csr Added DC5 and DC6 counter register for ICL in debugfs entry.

DC5 and DC6 counter register tells about residency of DC5 and DC6.
Added the same in debugfs file.

v2 : Remove csr_version check.
     Added generic check regarding DC counters for  Gen9 onwards. (Rodrigo)
v3 : Simplified gen checks. (Chris)
v4 : Simplified "if" ladder for multiple gens.
v5 : Removed unnecessary comment.

Signed-off-by: Jyoti Yadav <jyoti.r.yadav@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1538762926-4880-1-git-send-email-jyoti.r.yadav@intel.com