st/mesa: avoid vertex texture and sampler updates for 0 case

st/mesa: avoid vertex texture and sampler updates for 0 case

If we had no vertex textures or samplers previously and we have none now,
don't bother doing the enables dance.

I was profiling nexuiz on noop and noticed these two functions in the
profile, this drops their usage from 0.86% to 0.03% and 0.23% to 0.03%
for texture and samplers.

Signed-off-by: Dave Airlie <airlied@redhat.com>