meson: Make prefer-{crocus,iris} always take effect

As written this would require that the driver be built before we looked at the option. This is wrong because it affects code outside of the driver, it's in libGL's PCI ID table. This is sort of harmless for crocus at the moment, but for iris you would need to build it in order to remove it from the table; if you built just i965 and tried to run it against gen9, the libGL you just built would direct the loader to the iris driver you just didn't, and setup would fail, which is: goofy.