drm/i915: Drop check for non-NULL entry in llist_for_each_entry_safe

Since the next entry is an offset from a pointer, it can not be NULL.
For simplicity, drop the extra conditional before calling cond_resched()

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
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/20190530082358.13663-1-chris@chris-wilson.co.uk