drm/shmobile: removed optional dummy crtc mode_fixup function.

This patch set nukes all the dummy crtc mode_fixup implementations.
(made on top of Daniel topic/drm-misc branch)

Signed-off-by: Carlos Palminha <palminha@synopsys.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>