[mlir] Add config for PDL (#69927)

Make it so that PDL in pattern rewrites can be optionally disabled.

PDL is still enabled by default and not optional bazel. So this should
be a NOP for most folks, while enabling other to disable.

This only works with tests disabled. With tests enabled this still
compiles but tests fail as there is no lit config to disable tests that
depend on PDL rewrites yet.