i965/gen8: Use HALIGN_16 for single sample mcs buffers

i965/gen8: Use HALIGN_16 for single sample mcs buffers

The original code meant to do this, but was only checking num_samples == 1 to
figure out if a surface was fast clear capable. However, we can allocate single
sample miptrees with num_samples == 0 (when it's an internally created buffer).

This fixes a bunch of the piglit tests on gen8. Other gens should have been
fine.

Here is the order of events that allowed this to slip through:
t0: I wrote halign patches and tested them. These alignment assertions are for
   gen8 fast clear surfaces, basically.
t1: I pushed bogus perf patch which made fast clears never happen
t2: Reworked halign patches based on Chad's feedback and introduced the bug this
   patch fixes.
t2.5: I tested reworked patches, but assertion wasn't hit because of t1.
t3. Matt fixed issue in t1 which made fast clears happen here:
commit 22af95af8316f2888a3935cdf774ff0997b3dd42
Author: Matt Turner <mattst88@gmail.com>
Date:   Thu Jun 18 16:14:50 2015 -0700

    i965: Add missing braces around if-statement.

This logic should match that of the v1 of my halign patch series.

Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Matt Turner <mattst88@gmail.com>
Reported-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: Mark Janes <mark.a.janes@intel.com>