i965/vs: Fix flaky texture swizzling

i965/vs: Fix flaky texture swizzling

If any component used the ZERO or ONE swizzle, its corresponding member
in the `swizzle` array would never be initialized. We *mostly* got away
with this, except when that memory happened to contain a value that
clobbered another channel when combined using BRW_SWIZZLE4().

NOTE: This is a candidate for stable branches.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>