i965/vec4: Only zero out unused message components when there are any.

i965/vec4: Only zero out unused message components when there are any.

Otherwise, coordinates with four components would result in a MOV
with a destination writemask that has no channels enabled:

mov(8) g115<1>.F 0D { align16 WE_normal NoDDChk 1Q };

At best, this is stupid: we emit code that shouldn't do anything.
Worse, it apparently causes GPU hangs (observable with Chris's
textureGather test on CubeArrays.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: Chris Forbes <chrisf@ijw.co.nz>
Cc: mesa-stable@lists.freedesktop.org