drm/amdgpu: Fix hang on device removal.

If removing while commands in flight you cannot wait to flush the
HW fences on a ring since the device is gone.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210512142648.666476-13-andrey.grodzovsky@amd.com