drm/bridge/synopsys: dw-hdmi: Enable workaround for v2.12a

It turns out that even new DW HDMI controllers exhibits same magenta
line issues as older versions.

Enable workaround for v2.12a.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181104182705.18047-15-jernej.skrabec@siol.net