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_de_config" [drivers/gpu/drm/sun4i/sun4i-tcon.ko] undefined!
ERROR: "sun8i_tcon_top_set_hdmi_src" [drivers/gpu/drm/sun4i/sun4i-tcon.ko] undefined!
ERROR: "sun8i_tcon_top_of_table" [drivers/gpu/drm/sun4i/sun4i-tcon.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: cf77d79b4e29 ("drm/sun4i: tcon: Add another way for matching mixers with tcon")
Fixes: 0305189afb32 ("drm/sun4i: tcon: Add support for R40 TCON")
Reviewed-by: Daniel Vetter <[email protected]>
Tested-by: Jon Hunter <[email protected]>
Tested-by: Matt Hart <[email protected]>
Signed-off-by: Maxime Ripard <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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