mesa/st: toggle EXT_texture_norm16 based on format support

mesa/st: toggle EXT_texture_norm16 based on format support

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2556
Fixes: 7f467d4f738 ("mesa: GL_EXT_texture_norm16 extension plumbing")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3941>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3941>