drm/i915: make enable/disable_gt_powersave locking consistent

The enable functions grabbed dev->struct_mutex themselves, whereas
the disable functions expected dev->struct_mutex to be held by the
caller. Move the locking out to the (currently only) callsite of
intel_enable_gt_powersave to make this more consistent.

Originally this was prep work for future patches, but I've chased down
a totally wrong alley. Still, I think this is a sensible
clarification.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index fdca5b9..3fbc802 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -7174,7 +7174,9 @@
 {
 	intel_init_clock_gating(dev);
 
+	mutex_lock(&dev->struct_mutex);
 	intel_enable_gt_powersave(dev);
+	mutex_unlock(&dev->struct_mutex);
 
 	if (IS_IVYBRIDGE(dev))
 		ivb_pch_pwm_override(dev);