drm/i915/gen9: simplify DC toggling code

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
[fix line over 80 chars checkpatch WARN in gen9_set_dc_state() (imre)]
Link: http://patchwork.freedesktop.org/patch/msgid/1446657859-9598-8-git-send-email-imre.deak@intel.com