drivers: video: simple_panel: make simple panel independent of backlight

This patch updates the necessary Kconfigs to make simple panel
driver independent of backlight driver  and compiling backlight
related code in simple-panel driver conditionally to when user
has set CONFIG_BACKLIGHT.

Signed-off-by: Nikhil M Jain <n-jain1@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>