x86/platform/uv/BAU: Fix payload queue setup on UV4 hardware

The BAU on UV4 does not need to maintain the payload queue tail pointer. Do
not initialize the tail pointer MMR on UV4.

Note that write_payload_tail is not an abstracted BAU function since it is
an operation specific to pre-UV4 versions. Then we must switch on the UV
version to control its usage, for which we use uvhub_version rather than
is_uv*_hub because it is quicker/more concise.

Signed-off-by: Andrew Banman <abanman@sgi.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Mike Travis <travis@sgi.com>
Acked-by: Dimitri Sivanich <sivanich@sgi.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: akpm@linux-foundation.org
Cc: rja@sgi.com
Link: http://lkml.kernel.org/r/1474474161-265604-10-git-send-email-abanman@sgi.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>