drm/panel: rm68200: Add backlight dependency

Like many other panel drivers, this one fails to build when backlight
support is disabled:

drivers/gpu/drm/panel/panel-raydium-rm68200.o: In function `rm68200_probe':
panel-raydium-rm68200.c:(.text+0x14a): undefined reference to `devm_of_find_backlight'

This adds the appropriate dependency.

Note that while include/linux/backlight.h provides a stub inline when
backlight support is not enabled, this isn't enough to deal with the
case where backlight support is built as a module but the panel driver
is built-in, in which case linking will still fail as above.

One way to avoid this is to add a dependency such as this:

        depends on BACKLIGHT_CLASS_DEVICE || BACKLIGHT_CLASS_DEVICE=n

but that is rather complex and misses the point that the panel support
is mostly useless without backlight support.

Fixes: 2b7ed18b ("drm/panel: Add support for Raydium RM68200 panel driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[treding@nvidia.com: clarify the need for the dependency]
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313210015.3344380-1-arnd@arndb.de