i965/fs: Don't [un]spill multiple registers at a time in SIMD8 mode

i965/fs: Don't [un]spill multiple registers at a time in SIMD8 mode

I thought this would be a clever way to make spilling less expensive.
However, it appears that the oword read/write messages we are using for
spilling ignore the execution size and assume SIMD16 whenever working with
more than one register.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>