ARM: davinci: da850-evm: switch to using a fixed regulator for lcdc

Now that the da8xx fbdev driver supports power control with an actual
regulator, switch to using a fixed power supply for da850-evm.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>