nvc0/ir: fix textureGather with a single offset

nvc0/ir: fix textureGather with a single offset

Recent fix for non-const offsets broke the case of a single offset (vs 4
offsets). The later code relies on the offs array to contain null values
to tell whether they should be added onto the srcs list.

Fixes: 5239bd592 ("nvc0/ir: fix overwriting of value backing non-constant gather offset")
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: mesa-stable@lists.freedesktop.org