i965/gen8+: Add aux buffer alignment assertions

i965/gen8+: Add aux buffer alignment assertions

This helped find the incorrect HALIGN values from the previous patches.

v2: Add PRM references for assertions (Chad)

v3: Remove duplicated part of commit message, assert num_samples > 1, instead of
num_samples > 0. (Chad)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chad Versace <chad.versace@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>