drm/omap: displays: panel-dpi: add backlight dependency

The new backlight code causes a link failure when backlight
support itself is disabled:

drivers/gpu/drm/omapdrm/displays/panel-dpi.o: In function `panel_dpi_probe_of':
panel-dpi.c:(.text+0x35c): undefined reference to `of_find_backlight_by_node'

This adds a Kconfig dependency like we have for the other OMAP
display targets.

Fixes: 39135a30

 ("drm/omap: displays: panel-dpi: Support for handling backlight devices")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>