drm/i915: Disable runtime power management during shutdown

At least on some TGL platforms PUNIT wants to access some display HW
registers, but it doesn't handle display power management (disabling DC
states as required) and so this register access will lead to a hang. To
prevent this disable runtime power management for poweroff and reboot.

v2:
- Add code comment clarifying the requirement of display power states.
  (Ville)
Reported-and-tested-by: Khaled Almahallawy <khaled.almahallawy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Khaled Almahallawy <khaled.almahallawy@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210127181909.128094-1-imre.deak@intel.com

drivers/gpu/drm/i915/i915_drv.c

@@ -1047,6 +1047,8 @@ static void intel_shutdown_encoders(struct drm_i915_private *dev_priv)
 void i915_driver_shutdown(struct drm_i915_private *i915)
 {
 	disable_rpm_wakeref_asserts(&i915->runtime_pm);
+	intel_runtime_pm_disable(&i915->runtime_pm);
+	intel_power_domains_disable(i915);
 
 	i915_gem_suspend(i915);
 
@@ -1062,7 +1064,15 @@ void i915_driver_shutdown(struct drm_i915_private *i915)
 	intel_suspend_encoders(i915);
 	intel_shutdown_encoders(i915);
 
+	/*
+	 * The only requirement is to reboot with display DC states disabled,
+	 * for now leaving all display power wells in the INIT power domain
+	 * enabled matching the driver reload sequence.
+	 */
+	intel_power_domains_driver_remove(i915);
 	enable_rpm_wakeref_asserts(&i915->runtime_pm);
+
+	intel_runtime_pm_driver_release(&i915->runtime_pm);
 }
 
 static bool suspend_to_idle(struct drm_i915_private *dev_priv)