drm/meson: fix 1px pink line on GXM when scaling video overlay

Playing media with a resolution smaller than the crtc size requires the
video overlay to be scaled for output and GXM boards display a 1px pink
line on the bottom of the scaled overlay. Comparing with the downstream
vendor driver revealed VPP_DUMMY_DATA not being set [0].

Setting VPP_DUMMY_DATA prevents the 1px pink line from being seen.

[0] https://github.com/endlessm/linux-s905x/blob/master/drivers/amlogic/amports/video.c#L7869

Fixes: bbbe775e ("drm: Add support for Amlogic Meson Graphic Controller")
Suggested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230303123312.155164-1-christianshewitt@gmail.com

drivers/gpu/drm/meson/meson_vpp.c

@@ -100,6 +100,8 @@ void meson_vpp_init(struct meson_drm *priv)
 			       priv->io_base + _REG(VPP_DOLBY_CTRL));
 		writel_relaxed(0x1020080,
 				priv->io_base + _REG(VPP_DUMMY_DATA1));
+		writel_relaxed(0x42020,
+				priv->io_base + _REG(VPP_DUMMY_DATA));
 	} else if (meson_vpu_is_compatible(priv, VPU_COMPATIBLE_G12A))
 		writel_relaxed(0xf, priv->io_base + _REG(DOLBY_PATH_CTRL));