drm/i915/selftests: Use friendly request names for live_timeslice_rewind

Rather than mixing [012] and (A1, A2, B2) for the request indices, use
the enums throughout.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616185518.11948-2-chris@chris-wilson.co.uk