drm/i915/execlists: Reduce list_for_each_safe+list_safe_reset_next

After staring at the list_for_each_safe macros for a bit, our current
invocation of list_safe_reset_next in execlists_schedule() simply
reduces to list_for_each.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180102151235.3949-11-chris@chris-wilson.co.uk