drm/i915/psr: Timestamps for PSR entry and exit interrupts.

Timestamps are useful for IGT tests that trigger PSR exit and/or wait for
PSR entry.

v2: Removed seqlock (Ville)
    Removed erroneous warning in irq loop (Chris)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Jose Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180403212420.25007-4-dhinakaran.pandiyan@intel.com

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2686,6 +2686,13 @@ static int i915_edp_psr_status(struct seq_file *m, void *data)
 	}
 	mutex_unlock(&dev_priv->psr.lock);
 
+	if (READ_ONCE(dev_priv->psr.debug)) {
+		seq_printf(m, "Last attempted entry at: %lld\n",
+			   dev_priv->psr.last_entry_attempt);
+		seq_printf(m, "Last exit at: %lld\n",
+			   dev_priv->psr.last_exit);
+	}
+
 	intel_runtime_pm_put(dev_priv);
 	return 0;
 }