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

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