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