ac/nir/ngg: fix ngg_gs_clear_primflags crash

ac/nir/ngg: fix ngg_gs_clear_primflags crash

We get current_clear_primflag_idx_var==NULL when stream 0 output
number is known, so output_compile_time_known==true. But we also
need this variable when stream 1~3 output number is unknown or
vertex number is less than a primitive's needs.

Fixes: 60ac5dda82e1 ("ac: Add NIR lowering for NGG GS.")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23614>