aux/draw: fix memory leak related to ureg_get_tokens()

aux/draw: fix memory leak related to ureg_get_tokens()

Indeed, the function nir_to_tgsi() returns an ureg_get_tokens() allocated
object which is assigned locally. The ureg_get_tokens() allocated object
should be freed.

For instance, this issue is triggered with a llvm enabled lima,
"piglit/bin/gl-1.0-rendermode-feedback -auto -fbo":
Direct leak of 512 byte(s) in 1 object(s) allocated from:
    #0 0x7faeaa4500 in __interceptor_realloc (/usr/lib64/libasan.so.6+0xa4500)
    #1 0x7fa4a88f1c in tokens_expand ../src/gallium/auxiliary/tgsi/tgsi_ureg.c:239
    #2 0x7fa4a88f1c in get_tokens ../src/gallium/auxiliary/tgsi/tgsi_ureg.c:262
    #3 0x7fa4a900f4 in copy_instructions ../src/gallium/auxiliary/tgsi/tgsi_ureg.c:2079
    #4 0x7fa4a900f4 in ureg_finalize ../src/gallium/auxiliary/tgsi/tgsi_ureg.c:2129
    #5 0x7fa4a91dfc in ureg_get_tokens ../src/gallium/auxiliary/tgsi/tgsi_ureg.c:2206
    #6 0x7fa4b20a2c in nir_to_tgsi_options ../src/gallium/auxiliary/nir/nir_to_tgsi.c:4011
    #7 0x7fa4a0c914 in draw_create_vertex_shader ../src/gallium/auxiliary/draw/draw_vs.c:77

Fixes: b5e782f5f431 ("aux/draw: use nir_to_tgsi for draw shader in llvm path")
Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21924>