drm/i915: Fix WARN_ON() on UP machines

WARN_ON(!spin_is_locked()) is not a good idea on a UP system w/o
spinlock debugging. Use WARN_ON_SMP() instead.

This check has been added in

commit 8ba2d185
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Fri Apr 12 15:18:37 2013 +0300

    drm/i915: protect backlight registers and data with a spinlock
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_panel.c

@@ -332,7 +332,7 @@ static u32 i915_read_blc_pwm_ctl(struct drm_device *dev)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	u32 val;
 
-	WARN_ON(!spin_is_locked(&dev_priv->backlight.lock));
+	WARN_ON_SMP(!spin_is_locked(&dev_priv->backlight.lock));
 
 	/* Restore the CTL value if it lost, e.g. GPU reset */