drm/meson: fix primary plane disabling

The primary plane disable logic is flawed, when the primary plane is
disabled, it is re-enabled in the vsync irq when another plane is updated.

Handle the plane disabling correctly by handling the primary plane
enable flag in the primary plane update & disable callbacks.

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605141253.24165-2-narmstrong@baylibre.com