drm/i915: Make encoder->mode_set callbacks optional

For a bunch of reasons we want to move away from the ->mode_set
callbacks: All hw state setup needs to move into ->enable hooks (so
that DOMS can do runtime pm) and all the configuration setup needs to
move into the compute_config functions.

To start with this make the enocer->mode_set callback optional.

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