drm/i915: unify the definitions of the HDMI/SDVO register

Since they're all the same register, leave all the #defines at the
same place, organized by Gen and also specify which bits are used by
only a specific port or encoding.

Also remove a few unused duplicates and adjust indentation.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>