of: unittest: fix I2C dependency

The unittest fails to link if I2C or I2C_MUX is a loadable module:

  drivers/built-in.o: In function `selftest_i2c_mux_remove':
  unittest.c:(.text+0xb0ce4): undefined reference to `i2c_del_mux_adapter'

This changes the newly added IS_ENABLED() checks to use IS_BUILTIN()
instead, which evaluates to false if the other driver is a module.

Reported-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: d5e75500

 ("of: unitest: Add I2C overlay unit tests.")
Signed-off-by: Rob Herring <robh@kernel.org>