drm/nouveau: remove checks for return value of debugfs functions

As there is no need to check for the return value of debugfs_create_file
and drm_debugfs_create_files, remove unnecessary checks and error
handling in nouveau_drm_debugfs_init.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>