vulkan/sync: Rework asserts a bit

ANV currently smashes off the TIMELINE bit depending on whether or not the i915 interface supports them, triggering assert(!type->get_value). Instead of requiring ANV to smash off function pointers, let the extra function pointers through and then assert on the feature bits before the function pointers get used. This should give us roughly the same amount of assert protection while side-stepping the feature disabling problem.