drm/i915/lspcon: Switch back to PCON mode after output replug

At least a ParadTech PS175 LSPCON chip/firmware uses long instead of
short pulses to signal output unplug/plug events. This is contrary to
how branch devices normally work which use short HPD signaling. This
chip will also switch to LS mode after an unplug event, which could be
the consequence of the long HPD signaling semantics and an effort to
save power automatically. Because of this we'll fail to do AUX and
detect the output after a replug event.

To fix this make sure we are in PCON mode during connector detection.

v2:
- Switch the mode in the proper spot.

Cc: raptorteak@gmail.com
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98912
Reported-and-tested-by: raptorteak@gmail.com
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487776252-6288-1-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -4325,9 +4325,13 @@ intel_dp_short_pulse(struct intel_dp *intel_dp)
 static enum drm_connector_status
 intel_dp_detect_dpcd(struct intel_dp *intel_dp)
 {
+	struct intel_lspcon *lspcon = dp_to_lspcon(intel_dp);
 	uint8_t *dpcd = intel_dp->dpcd;
 	uint8_t type;
 
+	if (lspcon->active)
+		lspcon_resume(lspcon);
+
 	if (!intel_dp_get_dpcd(intel_dp))
 		return connector_status_disconnected;