i965/vs: Fix the mlen of scratch read/write messages.

i965/vs: Fix the mlen of scratch read/write messages.

These messages always have m0 = g0 and m1 = offset, and write has m2 = data.
Avoids regression in opt_compute_to_mrf() with a change to scratch writes to
set up the data as an MRF write in the IR.

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