drm/i915: DP link training optimization

This patch adds DP link training optimization by reusing the
previously trained values.

v2:
- rebase

V3:
- rebase

V4:
- when HPD long pulse is received, the flag is cleared
  that indicates if DP link training is required or not
  (based on Sivakumar's comment)
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -3794,7 +3794,7 @@ intel_dp_complete_link_train(struct intel_dp *intel_dp)
 	intel_dp->DP = DP;
 
 	if (channel_eq) {
-		intel_dp->train_set_valid = is_edp(intel_dp);
+		intel_dp->train_set_valid = true;
 		DRM_DEBUG_KMS("Channel EQ done. DP Training successful\n");
 	}
 }
@@ -4858,6 +4858,8 @@ intel_dp_hpd_pulse(struct intel_digital_port *intel_dig_port, bool long_hpd)
 	intel_display_power_get(dev_priv, power_domain);
 
 	if (long_hpd) {
+		/* indicate that we need to restart link training */
+		intel_dp->train_set_valid = false;
 
 		if (HAS_PCH_SPLIT(dev)) {
 			if (!ibx_digital_port_connected(dev_priv, intel_dig_port))