drm/amdgpu: add to custom amdgpu_drm_release drm_dev_enter/exit

User queues are disabled before GEM objects are released
(protecting against user app crashes).
No races with PCI hot-unplug (because drm_dev_enter prevents cleanup
if iewdevice is being removed).

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