drm/i915/execlists: Use list_safe_reset_next() instead of opencoding

list.h provides a macro for updating the next element in a safe
list-iter, so let's use it so that it is hopefully clearer to the reader
about the unusual behaviour, and also easier to grep.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161205142941.21965-6-chris@chris-wilson.co.uk