zaphod support broke the modesetting driver

[Mark Kettenis]

Commit 19e1dc8f6ea6d7ff5ba4a5caa0e2f40a47879408 broke the modesetting
driver quite badly on systems without XSERVER_PLATFORM_BUS.  The
problem is that the "entity private" only gets set up when
ms_platform_probe() gets called, but is used unconditionally in the
driver.  So on systems where ms_platform_probe() doesn't get called,
this results in a segmentation fault.

I tried moving the code that sets up the entity private around a bit,
but I couldn't get it to work as the entity private ends up being a
NULL pointer.