drm: fixes when i2c encoder slave mode_fixup is null.

Avoid i2c slave encoder drivers to copy/paste code to implement functions that will
only return true.

Signed-off-by: Carlos Palminha <palminha@synopsys.com>
[danvet: whitespace requested by Jani.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455106118-32145-1-git-send-email-palminha@synopsys.com