drm: simplify authentication management

The magic auth tokens we have are a simple map from cyclic IDs to drm_file
objects. Remove all the old bulk of code and replace it with a simple,
direct IDR.

The previous behavior is kept. Especially calling authmagic multiple times
on the same magic results in EINVAL except on the first call. The only
difference in behavior is that we never allocate IDs multiple times as
long as a client has its FD open.

v2:
 - Fix return code of GetMagic()
 - Use non-cyclic IDR allocator
 - fix off-by-one in "magic > INT_MAX" sanity check

v3:
 - drop redundant "magic > INT_MAX" check

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>