ARM: davinci_all_defconfig: enable GPIO backlight

DA850 EVM has been converted to use GPIO backlight device
for display backlight GPIO control.

Enable the GPIO backlight module in davinci_all_defconfig
to keep backlight support working.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
[nsekhar@ti.com: edits to commit message for context]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>