drm/sti: 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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/08d27e27582fb2daa48555ab542245c6cf0a2268.1455630967.git.palminha@synopsys.com