display: Move custom_brightness setter and update function from plugin to core displa...

display: Move custom_brightness setter and update function from plugin to core display module

Remove and redefine custom_brightness setter and update function.
These functions are added to below display-backlight.

void display_backlight_set_custom_brightness(int brightness)
-> This function set custom_brightness value.
int display_backlight_update_by_custom_brightness(void)
-> If the conditions are met, this function updates brightness with custom_brightness value.

Change-Id: I4c3662fbdfff4a4b17224ac3af74c4cac1503303
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>