drm/amdgpu: rename amdgpu_debugfs_preempt_cleanup

to amdgpu_debugfs_fini.  It will be used for other things in
the future.

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>