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: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>