drm/i915/lvds: Remove ->mode_set callback

All the hard work was already done, only thing left to do is remove
the empty callback. And a now rather misleading comment I've spotted
while reading through code.

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