drm/i915/opregion: fix build error on CONFIG_ACPI=n

Fix CONFIG_ACPI=n build fail

  CC      drivers/gpu/drm/i915/intel_opregion.o
drivers/gpu/drm/i915/intel_opregion.c: In function ‘intel_opregion_setup’:
drivers/gpu/drm/i915/intel_opregion.c:879:2: error: ‘asle_work’ undeclared (first use in this function)
drivers/gpu/drm/i915/intel_opregion.c:879:2: note: each undeclared identifier is reported only once for each function it appears in
make[4]: *** [drivers/gpu/drm/i915/intel_opregion.o] Error 1

introduced in
commit 91a60f2
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Thu Oct 31 18:55:48 2013 +0200

    drm/i915: move opregion asle request handling to a work queue

Reported-by: Jim Davis <jim.epost@gmail.com>
Reference: http://lkml.kernel.org/r/CA+r1ZhjcFpr5KKVX0pLCOP8cAyZoiYO=UyqYMJtNSV-Kt_p7xQ@mail.gmail.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_opregion.c

@@ -876,7 +876,9 @@ int intel_opregion_setup(struct drm_device *dev)
 		return -ENOTSUPP;
 	}
 
+#ifdef CONFIG_ACPI
 	INIT_WORK(&opregion->asle_work, asle_work);
+#endif
 
 	base = acpi_os_ioremap(asls, OPREGION_SIZE);
 	if (!base)