display: Move default_brightness getter and setter from plugin to core display module

display: Move default_brightness getter and setter from plugin to core display module

Remove and redefine default brightness getter/setter.
These functions are added below display-backlight.

int display_backlight_set_default_brightness(int brightness)
int display_backlight_get_default_brightness(int *brightness)
-> With these functions, it is possible to get and set default brightness value.

Change-Id: Iaa851094abf051f2fe15c64ff8e94376cc2b6382
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>