Gen8 uses incorrect MUX configuration for compute extended

[matt-auld]

I noticed this the other day while poking around, but for some reason forgot to post an issue, so while I still remember...

The generated configuration code for BDW in i915_oa_bdw.c:select_compute_extended_config incorrectly initialises the MUX registers for INTEL_INFO(dev_priv)->subslice_mask & 0x20.

I guess there is also the issue now of whether or not the mux_config_compute_extended is in fact needed at all...or maybe this should just go in the else?