drm/i915/skl: Expose DC5/DC6 entry counts

drm/i915/skl: Expose DC5/DC6 entry counts

The CSR firmware expose two counters, handy to check if we are indeed
entering DC5/DC6.

v2: Rebase
v3: Take RPM ref before reading (Imre)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> (v1)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446220412-32574-1-git-send-email-mika.kuoppala@intel.com
Tested-by: Daniel Stone <daniels@collabora.com> # SKL
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>