The PRM (Vol 1, p32) specifies that the URB_FENCE command must not cross
a cache-line boundary (64-bytes) in order to workaround a silicon issue.
Ensure that it does not by inserting an alignment point before the atomic
section.

This is a slightly too large hammer, but the easiest method to work with
the current BEGIN_BATCH/ADVANCE_BATCH protections.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>