drm/i915/dvo: switch ->mode_fixup to ->compute_config

This is the last encoder ->mode_fixup callback we have left, so
convert it.

Note that we want to only rip out the encoder->mode_fixup callback.
But we still have the dvo_slave->mode_fixup callback. dvo is gen2
only, so we won't ever touch this again. Hence why I didn't go through
all 6-7 dvo slave drivers and give them the same treatment. I'll add a
note to the commit message about this when merging, presuming there's
nothing else in the patch that needs to be fixed up.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Add note about why we keep the dvo->mode_fixup callback to
answer a question from Rodrigo's review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>