drm/i915/skl: enable PC9/10 power states during suspend-to-idle

During suspend-to-idle we need to keep the DMC firmware active and DC6
enabled, since otherwise we won't reach deep system power states like
PC9/10. The lead for this came from Nivedita who noticed that the
kernel's turbostat tool didn't report any PC9/10 residency change
across an 'echo freeze > /sys/power/state'.

Reported-by: Nivedita Swaminathan <nivedita.swaminathan@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447860750-18110-1-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/i915_drv.h

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 5a5c06b..3d8741ef 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1885,6 +1885,7 @@
 	u32 chv_phy_control;
 
 	u32 suspend_count;
+	bool suspended_to_idle;
 	struct i915_suspend_saved_registers regfile;
 	struct vlv_s0ix_state vlv_s0ix_state;