display: Add functions related to brightness

display: Add functions related to brightness

Move plugin static functions to core(display module).
This is for moving backlight_ops functions related to brightess without dependency.

int display_backlight_get_max_brightness(int *max_brightness)
-> This function replaces the get_max_brightness function located in plugins.

int display_backlight_get_normalized_brightness(int raw_brightness,
						int *normalized_brightness)
-> This function replaces the get_brt_normalized function located in plugins.

Change-Id: I7452f90075ef63acdc8f595fe99d47e0bfe4f4a1
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>