i965/gen8: Don't allocate hiz miptree structure

i965/gen8: Don't allocate hiz miptree structure

We now skip allocating a hiz miptree for gen8. Instead, we calculate
the required hiz buffer parameters and allocate a bo directly.

v2:
 * Update hz_height calculation as suggested by Topi
v3:
 * Bail if we failed to create the bo (Ben)
v4:
 * CEILING => DIV_ROUND_UP
 * Make sure mt->logical_depth0 being 0 would not cause trouble
 * Fail if Y tiling is not returned

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67564
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>