drm/sun4i: Initialize registers in tcon-top driver

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

drivers/gpu/drm/sun4i/sun8i_tcon_top.c

@@ -167,6 +167,13 @@ static int sun8i_tcon_top_bind(struct device *dev, struct device *master,
 		goto err_assert_reset;
 	}
 
+	/*
+	 * At least on H6, some registers have some bits set by default
+	 * which may cause issues. Clear them here.
+	 */
+	writel(0, regs + TCON_TOP_PORT_SEL_REG);
+	writel(0, regs + TCON_TOP_GATE_SRC_REG);
+
 	/*
 	 * TCON TOP has two muxes, which select parent clock for each TCON TV
 	 * channel clock. Parent could be either TCON TV or TVE clock. For now