drm/i915/gtt: add missing include file asm/smp.h

Fix build error:
drivers/gpu/drm/i915/gt/intel_ggtt.c: In function ggtt_restore_mappings:
drivers/gpu/drm/i915/gt/intel_ggtt.c:1239:3: error:
	implicit declaration of function wbinvd_on_all_cpus; did you mean wrmsr_on_cpus? [-Werror=implicit-function-declaration]
   wbinvd_on_all_cpus();
      ^~~~~~~~~~~~~~~~~~
         wrmsr_on_cpus
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200109012303.153001-1-chenzhou10@huawei.com

drivers/gpu/drm/i915/gt/intel_ggtt.c

@@ -6,6 +6,7 @@
 #include <linux/stop_machine.h>
 
 #include <asm/set_memory.h>
+#include <asm/smp.h>
 
 #include "intel_gt.h"
 #include "i915_drv.h"