drm/nouveau: keep DMA buffers required for suspend/resume

Nouveau deallocates a few buffers post GPU init which are required for GPU suspend/resume to function correctly.
This is likely not as big an issue on systems where the NVGPU is the only GPU, but on multi-GPU set ups it leads to a regression where the kernel module errors and results in a system-wide rendering freeze.

This commit addresses that regression by moving the two buffers required for suspend and resume to be deallocated at driver unload instead of post init.

Fixes: 042b5f83 ("drm/nouveau: fix several DMA buffer leaks")
Signed-off-by: Sid Pranjale <sidpranjale127@protonmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>