Pollable spin batch exports a spin->running pointer which can be checked
by dereferencing it to see if the spinner is actually executing on the
GPU.

This is useful for tests which want to make sure they do not proceed with
their next step whilst the spinner is potentially only being processed by
the driver and not actually executing.

Pollable spinner can be created with igt_spin_batch_new_poll or
__igt_spin_batch_new_poll, after which igt_spin_busywait_until_running can
be used to busy wait until it is executing.

v2:
 * Move READ_ONCE to igt_core.
 * Add igt_spin_busywait_until_running. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>