If I allocate too much memory I get a kernel hang using the drm-next-4.8 branch
from Alex Deucher. SSH stops working too.

This has been tests by allocating GTT buffers in a loop using the winsys for
the new radv vulkan driver. Just as radeonsi, the winsys has a 3 step
allocation process: 1) allocate the buffer 2) Allocate an address range 3) Map
the buffer to address range.

This hang is a regression. Bisection pointed to the following patch as culprit:

commit 089f16c55baacd5e8ae3745625efa82899b4b217
Author: Christian König <christian.koenig@amd.com>
Date:   Mon Jun 6 10:17:50 2016 +0200

    drm/ttm: cleanup ttm_tt_(unbind|destroy)

    ttm_tt_destroy should be the only one unbinding the object.

    Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
    Signed-off-by: Christian König <christian.koenig@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>