drm/i915/gen9: Check for DC state mismatch

The DMC can incorrectly run off and allow DC states on it's own. We
don't know the root-cause for this yet but this patch makes it more
visible.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455808874-22089-2-git-send-email-mika.kuoppala@intel.com