The set of available structs is not protected by the spinlock, and for
the single read we can use READ_ONCE instead.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323101944.21627-7-chris@chris-wilson.co.uk