modes: Retry GETCONNECTOR if a hotplug event occurs between the two ioctls

If the available modes changes between the two GETCONNECTOR ioctls, that
caused the kernel to skip filling one array and led to a crash (as the size
of the allocated and initialised block of memory differed from the reported
size, and might be NULL if no modes were present at first).

This bug manifest its self on my machine due to spurious false positive
detections of a connected TV-out.

Fixes: http://bugs.freedesktop.org/show_bug.cgi?id=25912


       Crash whilst probing modes

Based upon the similar fixes for the GETRESOURCES ioctls by Chris Wilson,
in the following commits:

    commit e6c136ca
    commit 85fb3e55
    commit d1308f4f

Signed-off-by: Peter Clifton <pcjc2@cam.ac.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>