drm/tilcd: select BACKLIGHT_LCD_SUPPORT

The dependecies for BACKLIGHT_CLASS_DEVICE are defined a bit
strange, but it seems one has to always select both BACKLIGHT_CLASS_DEVICE
and BACKLIGHT_LCD_SUPPORT to avoid this error:

drivers/gpu/drm/tilcdc/tilcdc_panel.c:396:
 undefined reference to `of_find_backlight_by_node'

Cc: Rob Clark <robdclark@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@gmail.com>