drm/i915: Don't initialize power seqeuencer delays more than once

Since we read the current power seqeuncer delays from the registers
(as well as looking at the vbt and spec values) we may end up
corrupting delays we already initialized when we switch to another
pipe and the power seqeuncer there has different values currently
in the registers.

So make sure we only initialize the delays once even if
intel_dp_init_panel_power_sequencer() gets called multiple times.

There was some discussion in the review about when exactly we need to
unlock the pps. Quoting Bspec:

"If this bit is not a zero, it activates the register write protect
 and writes to those registers will be ignored unless the write
 protect key value is set in the panel sequencing control register."
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Add Bspec quote per review discussion between Imre and
Ville.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -4733,6 +4733,10 @@ intel_dp_init_panel_power_sequencer(struct drm_device *dev,
 
 	lockdep_assert_held(&dev_priv->pps_mutex);
 
+	/* already initialized? */
+	if (final->t11_t12 != 0)
+		return;
+
 	if (HAS_PCH_SPLIT(dev)) {
 		pp_ctrl_reg = PCH_PP_CONTROL;
 		pp_on_reg = PCH_PP_ON_DELAYS;