This test forgot to specify the existing CRTC routings in the setup. For the
first output the default 0 was ok, now it is -1 to ensure that the code will
assign it correctly. For the second output the default 0 was incorrect, because
possible_crtcs does not include 0.  Now that CRTC is initialized to off
instead, because the second output is hotplugged later and running a CRTC
without an output does not make sense.

This fix will keep this test passing when a future patch attempts to preserve
existing CRTC routings. Assuming that any existing routing is valid, such
routing will be kept. In this test case the existing routing was illegal, it
should have been impossible, which then causes that future patch to fail the
test by assigning the wrong CRTC.

#373