drm/i915: move HDMI structs to shared location

Move intel_hdmi data structure and support functions to a shared location,
to allow their usage from intel_ddi module.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>