drm/radeon: fix S/R on VM systems (cayman/TN/SI)

We weren't properly tearing down the VM sub-alloctor
on suspend leading to bogus VM PTs on resume.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=60439



Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Dmitry Cherkasov <Dmitrii.Cherkasov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org