drm/i915: Apply headless DMC workaround for CNL

With firmware 1.07 having fixed the state corruption issue, we can enable
the headless GT performance workaround for CNL as well. (Equivalent to
b687637 ("drm/i915: Restore GT performance in headless mode with DMC
loaded") on other affected platforms.)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100572
Testcase: igt/gem_exec_nop/headless
Cc: Imre Deak <imre.deak@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111082417.795-1-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/intel_runtime_pm.c

@@ -1848,6 +1848,7 @@ void intel_display_power_put(struct drm_i915_private *dev_priv,
 	BIT_ULL(POWER_DOMAIN_INIT))
 #define CNL_DISPLAY_DC_OFF_POWER_DOMAINS (		\
 	CNL_DISPLAY_POWERWELL_2_POWER_DOMAINS |		\
+	BIT_ULL(POWER_DOMAIN_GT_IRQ) |			\
 	BIT_ULL(POWER_DOMAIN_MODESET) |			\
 	BIT_ULL(POWER_DOMAIN_AUX_A) |			\
 	BIT_ULL(POWER_DOMAIN_INIT))