fbdev: da8xx: remove panel_power_ctrl() callback from platform data

There are no more users of panel_power_ctrl(). Remove it from the
driver.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>