drm/bridge: removed dummy mode_fixup function from dw-hdmi.

Other bridge drivers don't implement this optional function.
Removed dummy code from dw-hdmi brigde driver.

Signed-off-by: Carlos Palminha <palminha@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455120639-29934-1-git-send-email-palminha@synopsys.com