drm/i915: clarify preferred sdvo input mode code

- kill intel_sdvo->input_dtd, it's only used as a temporary variable,
  we store the preferred input mode in the adjusted mode at mode_fixup
  time.
- rename the function to make it clear what we want it to do (get the
  preferred mode) and say in a comment what it unfortunately does as a
  side-effect (set the new output timings).

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>