drm/i2c/tda998x: removed unnecessary code, mode_fixup is now optional.

Removed dummy mode_fixup.

Signed-off-by: Carlos Palminha <palminha@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455118167-12178-3-git-send-email-palminha@synopsys.com