drm/i915: use drm_modeset_lock_all

Two exceptions:
- debugfs files only read information which is not related to crtc, so
  can stay on the modeset_config lock.
- Same holds for the edp vdd work in intel_dp.c. Add a corresponding
  WARN_ON and a comment next to the intel_dp struct fields for
  documentation.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -1057,6 +1057,8 @@ static void ironlake_panel_vdd_off_sync(struct intel_dp *intel_dp)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	u32 pp;
 
+	WARN_ON(!mutex_is_locked(&dev->mode_config.mutex));
+
 	if (!intel_dp->want_panel_vdd && ironlake_edp_have_panel_vdd(intel_dp)) {
 		pp = ironlake_get_pp_control(dev_priv);
 		pp &= ~EDP_FORCE_VDD;