drm/i915: drop helper vtable for sdvo encoder

Completely unused by now. Separate patch in case I've missed a
place somewhere which dereferences the helper vtable but actually
shouldn't do so.

v2: Resolve rebase conflict with Egbert Eich's hpd infrastructure
rework.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>