Fixes the infamous 'runtime PM' bug many users are facing on Laptops with
Nvidia Pascal GPUs by skipping said PCI power state changes on the GPU.
Depending on the used kernel there might be messages like those in demsg:
"nouveau :01:00.0: Refused to change power state, currently in D3"
"nouve
Since 987d65d01356 (drm: debugfs: make
drm_debugfs_create_files() never fail), there is no need to ever check
for the the return value of debugfs_create_file() and
drm_debugfs_create_files(). Therefore, remove unnecessary checks and
error handling in nouveau_drm_debugfs_init() and have the function
