drm/meson: fix G12A primary plane disabling

The G12A Primary plane was disabled by writing in the OSD1 configuration
registers, but this caused the plane blender to stall instead of continuing
to blend only the overlay plane.

Fix this by disabling the OSD1 plane in the blender registers, and also
enabling it back using the same register.

Fixes: 490f50c1 ("drm/meson: Add G12A support for OSD1 Plane")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
[narmstrong: fixed nit in commit log]
Link: https://patchwork.freedesktop.org/patch/msgid/20190605141253.24165-3-narmstrong@baylibre.com