Track the position of the HWSP for each timeline.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/2169Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200714135002.17508-1-chris@chris-wilson.co.uk