drm/i915/pch: Save/restore PCH_PORT_HOTPLUG across suspend

At least on a Lenovo X220 the HPD bits of this are enabled at boot but
cleared after resume, which means plug interrupts stop working.

This also happens to fix DP displays re-lighting on resume.  I'm quite
certain that's an accident: the first DP link train inevitably fails on
that machine, and it's only serendipity that we're getting multiple plug
interrupts and the second train works.  But I shall take my victories
where I get them.
Signed-off-by: Adam Jackson <ajax@redhat.com>
Tested-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -541,6 +541,7 @@ typedef struct drm_i915_private {
 	u32 savePIPEB_LINK_M1;
 	u32 savePIPEB_LINK_N1;
 	u32 saveMCHBAR_RENDER_STANDBY;
+	u32 savePCH_PORT_HOTPLUG;
 
 	struct {
 		/** Bridge to intel-gtt-ko */

drivers/gpu/drm/i915/i915_suspend.c

@@ -814,6 +814,7 @@ int i915_save_state(struct drm_device *dev)
 		dev_priv->saveFDI_RXB_IMR = I915_READ(_FDI_RXB_IMR);
 		dev_priv->saveMCHBAR_RENDER_STANDBY =
 			I915_READ(RSTDBYCTL);
+		dev_priv->savePCH_PORT_HOTPLUG = I915_READ(PCH_PORT_HOTPLUG);
 	} else {
 		dev_priv->saveIER = I915_READ(IER);
 		dev_priv->saveIMR = I915_READ(IMR);
@@ -865,6 +866,7 @@ int i915_restore_state(struct drm_device *dev)
 		I915_WRITE(GTIMR, dev_priv->saveGTIMR);
 		I915_WRITE(_FDI_RXA_IMR, dev_priv->saveFDI_RXA_IMR);
 		I915_WRITE(_FDI_RXB_IMR, dev_priv->saveFDI_RXB_IMR);
+		I915_WRITE(PCH_PORT_HOTPLUG, dev_priv->savePCH_PORT_HOTPLUG);
 	} else {
 		I915_WRITE(IER, dev_priv->saveIER);
 		I915_WRITE(IMR, dev_priv->saveIMR);