i965/hiz: Make void pointer type casts explicit

i965/hiz: Make void pointer type casts explicit

Previous to this patch, gen6_hiz.c contained two implicit type casts
from void * to a a non-void pointer type.  This is allowed in C but
not in C++.  This patch makes the type casts explicit, so that
gen6_hiz.c can be converted into a C++ file.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>