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

Peter Clifton hit an issue whereby he had a spurious TV hotplug event
that occurred between the two GETRESOURCES 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).

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


       Crash whilst probing modes

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