drm/radeon: constify more struct drm_*_helper funcs pointers

Some non-const pointers were added since the last constification, fix
them.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>