drm/i915: document why dvo/sdvo/crt need a special dpms function

In the cloned case, changing just one output but keeping the other, the
pipe state won't change and intel_crtc_update_dpms will be a nop, but we
still need to update the dpms state of the output being changed.

Only dvo, sdvo and crt are cloneable, so only those three have special
dpms functions.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>