drm: sanitise drm modesetting API + remove unused hotplug

The initially merged modesetting API has some uglies in it, this
cleans up the struct members and ioctl ordering for initial submission.

It also removes the unneeded hotplug infrastructure.

airlied:- I've pulled this patch in from git modesetting-gem tree.

Signed-off-by: Dave Airlie <airlied@redhat.com>