Channel 1 has polarity bits for vsync and hsync signals but driver never
sets them. It turns out that with pre-HDMI2 controllers seemingly there
is no issue if polarity is not set. However, with HDMI2 controllers
(H6) there often comes to de-synchronization due to phase shift. This
causes flickering screen. It's safe to assume that similar issues might
happen also with pre-HDMI2 controllers.

Solve issue with setting vsync and hsync polarity. Note that display
stacks with tcon top have polarity bits actually in tcon0 polarity
register.

Fixes: 9026e0d1 ("drm: Add Allwinner A10 Display Engine support")
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Tested-by: Andre Heider <a.heider@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209175900.7092-3-jernej.skrabec@siol.net