i965: Fix stride field for the result of emit_uniformize().

i965: Fix stride field for the result of emit_uniformize().

This is essentially the same problem fixed in an earlier patch for
immediates.  Setting the stride to zero will be particularly useful
for my future SIMD lowering pass, because we will be able to just
check whether the stride of a source register is zero and skip
emitting the copies required to unzip it in that case.

Instead of setting stride to zero in every caller of emit_uniformize()
I've changed the function to return the result as its return value
(previously it was being written into a caller-provided destination
register), because this way we can enforce that the result is used with
the correct regioning from the function itself.

The changes to the prototype of its VEC4 counterpart are mainly for
the sake of symmetry, VEC4 registers don't have stride.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>