iris: Fix handling of SIMD32 fragment shaders

The brw_wm_prog_data_dispatch_grf_start_reg() and _prog_offset() helpers read the _NPixelDispatchEnable fields from 3DSTATE_PS to figure out which bits to pull out of the prog data and stuff where. Therefore, they need to be called with the final set of _NPixelDispatchEnable bits after we've done the workaround for SIMD32 and 16x MSAA. Otherwise, if you end up with a somewhat odd combination of enables, the GRF start reg and KSP data ends up in the wrong slots. In particular, running SIMD32-only is broken but several other combinations are as well.