i965/fs: Use correct spill offsets

i965/fs: Use correct spill offsets

Different platforms require the offset to be in different units.  However,
the generator fixes all of this up for us and only requires an offset in
bytes.  Previously, we were getting this wrong all over the place.  Some
computed/used it correctly as bytes while others treated the offset as
whole registers or computed it as bytes or bytes*2 in SIMD16 mode.  This
commit cleans all this up and makes us properly treat it as bytes
everywhere.

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