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: NImre Deak <imre.deak@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -7210,6 +7210,8 @@ static int intel_crtc_mode_set(struct drm_crtc *crtc,
 			encoder->base.base.id,
 			drm_get_encoder_name(&encoder->base),
 			mode->base.id, mode->name);
+
+		if (encoder->mode_set)
 			encoder->mode_set(encoder);
 	}