drm/i915/sdvo: Remove ->mode_set callback

SDVO is used by both crtcs using the i9xx_ and the ironlake_
functions. For both cases there is nothing between the
encoder->mode_set and the encoder->pre_enable calls that touches the
hardware.

The vlv_ functions are different since they enable the pll before the
->pre_enable hook. But SDVO isn't supported on vlv platforms, so this
doesn't matter.

We've also already clean up all the sdvo state computation logic, all
relevant parts are already in the ->compute_config hook.  So we can
just get rid of the ->mode_set hook by converting it to a ->pre_enable
hook.

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