mesa: fix refcnt imbalance related to egl_image_target_texture()

mesa: fix refcnt imbalance related to egl_image_target_texture()

Indeed, the locally allocated "stimg" reference was not freed
on a specific code path.

For instance, this issue is triggered on radeonsi or r600 with:
"piglit/bin/egl-ext_egl_image_storage -auto -fbo"
while setting GALLIUM_REFCNT_LOG=refcnt.log.

Fixes: 6a3f5c65128b ("mesa: simplify st_egl_image binding process for texture storage")
Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23165>