intel/fs,vec4: Use g0 as the header for MFENCE

intel/fs,vec4: Use g0 as the header for MFENCE

We set header_present but then pass it some random garbage.  Give it g0
instead.  I'm not actually sure this does anything but g0 is the usual
header data and this is what the windows driver does so it seems like a
good idea.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>