drm/i915/ddi: Remove ->mode_set callback

A bit more care required here since there are some very few things
between the call to encoder->mode_set and encoder->pre_enable. But
they're either book-keeping or only matter for the vga port on the
pch. So of no concern.

Note that with the new sequence we write the infoframes after
selecting the clock source, but that shouldn't matter. I've simply
opted for this to have simpler code.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>