drm/i915/dmabuf: fix broken build

wbinvd_on_all_cpus() is only defined on x86 it seems, plus we need to
include asm/smp.h here.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211021125332.2455288-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c

@@ -12,6 +12,13 @@
 #include "i915_gem_object.h"
 #include "i915_scatterlist.h"
 
+#if defined(CONFIG_X86)
+#include <asm/smp.h>
+#else
+#define wbinvd_on_all_cpus() \
+	pr_warn(DRIVER_NAME ": Missing cache flush in %s\n", __func__)
+#endif
+
 I915_SELFTEST_DECLARE(static bool force_different_devices;)
 
 static struct drm_i915_gem_object *dma_buf_to_obj(struct dma_buf *buf)