drm/i915/selftests: Extract spinner code

Pull out spinner code to a standalone file to enable it to be shortly used
by other and new test cases.

Plain code movement - no functional changes.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181130080254.15383-1-tvrtko.ursulin@linux.intel.com