drm/nvc0/fence: restore pre-suspend fence buffer context on resume

Fixes some unfortunate races on resume.  The G84 version of the code doesn't
need this as "gpuobj"s are automagically suspended/resumed by the core code
whereas pinned buffer objects are not.

Cc: stable@vger.kernel.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>