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

In the Linux kernel, the following vulnerability has been resolved: 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.