i965/fs: Fix message setup for SIMD8 spills.

i965/fs: Fix message setup for SIMD8 spills.

In the SIMD16 spilling changes, I replaced a "1" in the spill path with
"mlen", but obviously it wasn't mlen before because spills have the g0
header along with the payload. The interface I was trying to use was
asking for how many physical regs we're writing, so we're looking for "1"
or "2".

I'm guessing this actually passed piglit because the high 8 bits of the
execution mask in SIMD8 mode are all 0s.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>