drm/sun4i: use sun4i_tcon_of_table to check if a device node is a TCON

The sun4i DRM driver maintains a list of compatible strings it uses to
check if a device node within the display component graph is a TCON.
The TCON driver also has this list, used to bind the TCON driver to
the device. These two lists are identical.

Instead of maintaining two identical lists, export the list from the
TCON driver for the DRM driver to use.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171127084632.25511-1-wens@csie.org