drm/i915: Hold rpm wakeref for modifying the global seqno

To modify the global seqno may require rewriting a few registers, which
requires us to hold the rpm wakeref. We must therefore take it around
the call to i915_gem_set_global_seqno() in debugfs, on behalf of the
user.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180102151235.3949-15-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -988,7 +988,10 @@ i915_next_seqno_set(void *data, u64 val)
 	if (ret)
 		return ret;
 
+	intel_runtime_pm_get(dev_priv);
 	ret = i915_gem_set_global_seqno(dev, val);
+	intel_runtime_pm_put(dev_priv);
+
 	mutex_unlock(&dev->struct_mutex);
 
 	return ret;