i965: Mask out unused Align16 components in brw_untyped_atomic.

i965: Mask out unused Align16 components in brw_untyped_atomic.

This is currently not a problem because the vec4 visitor happens to
mask out unused components from the destination, but it might become
an issue when we start using atomics without writeback message.  In
any case it seems sensible to set it again here because the
consequences of setting the wrong writemask (random graphics memory
corruption) are difficult to debug and can easily go unnoticed.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>