i965/fs: Remove bogus field prog_data->dispatch_width.

i965/fs: Remove bogus field prog_data->dispatch_width.

Despite the name, this field wasn't being set to the dispatch width at
all; it was always 8.  The only place it was used was that the
constant buffer read length was aligned to it, and as far as I can
tell from the docs, there is no need to align this value to the
dispatch width; aligning it to a multiple of 8 is sufficient.  So I've
just replaced it with a hardcoded 8.

v2: In gen6_wm_state, use brw->wm.base.push_const_size for consistency
with VS and GS state upload.

Reviewed-by: Eric Anholt <eric@anholt.net>