gk104/ir: simplify and fool-proof texbar algorithm

gk104/ir: simplify and fool-proof texbar algorithm

With the current algorithm, we only look at tex uses. However there's a
write-after-write hazard where we might decide to, on some path, not use
a texture's output at all, but instead to write a different value to
that register. However without the barrier, the texture might complete
later and overwrite that value.

This fixes Unreal Elemental demo on GK110/GK208, flightgear on GK10x,
and likely other random-looking failures.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "11.1" <mesa-stable@lists.freedesktop.org>