Initialize cursor support on GPU hotplug
This is a necessary fix for having a hotplugged DisplayLink device use hardware cursors.
When a DisplayLink USB 3 dock is hotplugged for the first time after a reboot, the EVDI driver will create a new DRM device (and a device node). Creating the DRM device is the GPU hotplug. The problem fixed by this MR does not occur if gnome-shell is restarted after the hotplug (log out and log in), or if EVDI for some reason had already created the DRM device nodes.
I originally wrote this fix for the 3.34 branch, where this is enough to get hardware cursors used. On master branch this fix is necessary, but not enough (I believe at least !1079 (merged) is needed as well, though I have yet to test it). I verified with temporary debug prints in should_have_hw_cursor ()
that this MR works.