drm: fixes crct set_mode when encoder mode_fixup is null.

Avoids null crash when encoders don't implement mode_fixup.

Signed-off-by: Carlos Palminha <palminha@synopsys.com>
[danvet: Also update kerneldoc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455106522-32307-1-git-send-email-palminha@synopsys.com