drm/radeon: cleanup debugfs

Remove unnecessary debugfs dentries and also radeon_ttm_debugfs_fini()
as drm_debugfs_cleanup() will recursively remove debugfs files.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>