drm/sun4i: Initialize registers in tcon-top driver

[ Upstream commit c96d62215fb540e2ae61de44cb7caf4db50958e3 ]

It turns out that TCON TOP registers in H6 SoC have non-zero reset
value. This may cause issues if bits are not changed during
configuration.

To prevent that, initialize registers to 0.

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/20181104182705.18047-24-jernej.skrabec@siol.net


Signed-off-by: Sasha Levin <sashal@kernel.org>