drm/radeon: Actually enable page flipping!

There was a left-over "return" at the beginning of
radeon_crtc_page_flip(), make page flipping a no-op...

This return statement was added when the function was not ready to build
yet and the code was #ifdef'd out. But when the #ifdef was removed, the
return statement remained there...

This fixes KDE startup when desktop effects are enabled.