drm/i915: move sdvo TV clock computation to intel_sdvo.c

We have a very nice infrastructure for this now!

Note that the multifunction sdvo support is pretty neatly broken: We
completely ignore userspace's request for which connector to wire up
with the encoder and just use whatever the last detect callback has
seen.

Not something I'll fix in this patch, but unfortunately something
which is also broken in the DDI code ...

v2: Don't call sdvo_tv_clock twice.

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