i965: Set subslice_total on Gen7/7.5 platforms.
i965: Set subslice_total on Gen7/7.5 platforms.

We'll use this for compute shader thread counts and scratch space
calculations shortly.

Note that subslices are referred to as "half slices" on Ivybridge.

Cc: "12.0" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Francisco Jerez <5906ff32bdd9fd8db196f6ba5ad3afd6f9257ea5@riseup.net>
Reviewed-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler