st/glsl_to_tgsi: simpler fixup of empty writemasks

st/glsl_to_tgsi: simpler fixup of empty writemasks

Empty writemasks mean "copy everything", so we can always just use the number
of vector elements (which uses the GLSL meaning here, i.e. each double is a
single element/writemask bit).

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Dave Airlie <airlied@redhat.com>