drm/i915: Fix edp vdd locking

Introduce a new mutex (pps_mutex) to protect the power sequencer
state. For now this state includes want_panel_vdd as well as the
power sequencer registers.

We need a single mutex (as opposed to per port) because later on we
will need to deal with VLV/CHV which have multiple power sequencer
which can be reassigned to different ports.

v2: Add the locking to intel_dp_encoder_suspend too (Imre)
v3: Take care intel_edp_backlight_power() and
    _intel_edp_backlight_on/off(), deal with reboot notifier
    vlv_power_sequencer_pipe() call (Imre)

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -1527,6 +1527,9 @@ struct drm_i915_private {
 	/* LVDS info */
 	bool no_aux_handshake;
 
+	/* protects panel power sequencer state */
+	struct mutex pps_mutex;
+
 	struct drm_i915_fence_reg fence_regs[I915_MAX_NUM_FENCES]; /* assume 965 */
 	int fence_reg_start; /* 4 if userland hasn't ioctl'd us yet */
 	int num_fence_regs; /* 8 on pre-965, 16 otherwise */

drivers/gpu/drm/i915/intel_display.c

@@ -12559,6 +12559,8 @@ static void intel_init_display(struct drm_device *dev)
 	}
 
 	intel_panel_init_backlight_funcs(dev);
+
+	mutex_init(&dev_priv->pps_mutex);
 }
 
 /*