drm/sun4i: hdmi: Fix incorrect assignment in sun4i_tmds_determine_rate

best_div is set to i which corresponds to rate halving when it should be
set to j which corresponds to the divider.

Fixes: 9c568101

 ("drm/sun4i: Add HDMI support")
Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180109020323.11852-3-net147@gmail.com