graphics/mesa-devel: stop nesting options after r537515

Adding _SINGLE/_MULTI in _DEFINE allows to disable all child options
if the parent option is disabled. However, if only parent is disabled
then the behavior is undefined.

$ __MAKE_CONF= PORT_DBDIR=/var/empty make WITHOUT=DRI -V MESON_ARGS:M\*dri-driver\*
-Ddri-drivers=i965

PR: 253741
Reported by: Evgeniy Khramtsov