freedreno/ir3: only tex instructions have wrmask

freedreno/ir3: only tex instructions have wrmask

At the ir3 level, we would assume that we could use wrmask to mask
off other components of an instruction returning a vecN when they are
not used.  Which would let RA use components not written for other live
values.  But this is only true for tex instructions.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Eric Anholt <eric@anholt.net>