drm/amdgpu: don't leave dangling pointers around

Right now it's possible to trigger fence_drv.fences[] dereference after
the array has been freed. While the real problem is elsewhere, this still
results in confusing errors that depend on how the freed memory was
reused (I've seen "kernel tried to execute NX-protected page"), it's
better to clear them and get NULL dereference so that it's obvious what's
going wrong.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>