drm/panel: add backlight dependency for sitronix-st7789v

Without the dependency, we run into a link error:

drivers/gpu/drm/panel/panel-sitronix-st7789v.o: In function `st7789v_probe':
panel-sitronix-st7789v.c:(.text.st7789v_probe+0xc0): undefined reference to `of_find_backlight_by_node'

Fixes: 7142afb3 ("drm/panel: Add driver for sitronix ST7789V LCD controller")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170419180326.303994-1-arnd@arndb.de