drm/sun4i: fix build failure with CONFIG_DRM_SUN8I_MIXER=m

Having DRM_SUN4I built-in but DRM_SUN8I_MIXER as a loadable module results in
a link error, as we try to access a symbol from the sun8i_tcon_top.ko module:

ERROR: "sun8i_tcon_top_of_table" [drivers/gpu/drm/sun4i/sun8i-drm-hdmi.ko] undefined!
ERROR: "sun8i_tcon_top_of_table" [drivers/gpu/drm/sun4i/sun4i-drm.ko] undefined!

This solves the problem by adding a silent symbol for the tcon_top module,
building it as a separate module in exactly the cases that we need it,
but in a way that it is reachable by the other modules.

Fixes: 57e23de0 ("drm/sun4i: DW HDMI: Expand algorithm for possible crtcs")
Fixes: ef0cf644 ("drm/sun4i: Add support for traversing graph with TCON TOP")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180711144403.1022829-1-arnd@arndb.de