i965/vs: Trim the swizzle of the scratch write temporary.

i965/vs: Trim the swizzle of the scratch write temporary.

This fixes confusion by the upcoming live variable analysis which saw e.g. use
of temp.w when only temp.xyz were initialized in the basic block, and
concluded that temp.w must have come from outside of the block (even though it
was never initialized anywhere).

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