drm/i915/execlists: Simply walk back along request timeline on reset

The request's timeline will only contain requests from this context, in
order of execution. Therefore, we can simply look back along this
timeline to find the currently executing request.

If we do find that the current context has completed its last request,
that does not imply that all requests are completed in the context, so
only advance the ring->head up to the end of the known completions!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028124125.25176-1-chris@chris-wilson.co.uk