linux: Make platform device probe less fragile

At the point where xf86BusProbe runs we haven't yet taken our own VT,
which means we can't perform drm "master" operations on the device. This
is tragic, because we need master to fish the bus id string out of the
kernel, which we can only do after drmSetInterfaceVersion, which for
some reason stores that string on the device not the file handle and
thus needs master access.

Fortunately we know the format of the busid string, and it happens to
almost be the same as the ID_PATH variable from udev. Use that instead
and stop calling drmSetInterfaceVersion.

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>