And make sure we get the LEASE=1 value, indicating a lessee change.

v2: Apparently netlink reading can leak EAGAIN out through
udev_monitor_receive_device. No idea what's going on there, so let's
wrap some duct tape around it.

v3: Lyude reported that we might get a few udev events on startup of
the test. Drain those first.

v4: Use the igt hotplug library functions, they already take care of
all the uevent special cases.

Cc: Lyude Paul <lyude@redhat.com>
Cc: Keith Packard <keithp@keithp.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>