display: signal: Relocate display_brightness_changed() to reduce plugin duplication

display: signal: Relocate display_brightness_changed() to reduce plugin duplication

Remove and redefine display_brightness_changed function in plugin/display/core.c
-> This function emits brightness signal, when the brightness is set.

This function is added to below display-signal.
void display_signal_register_display_brightness_notifier(void)
void display_signal_unregister_display_brightness_notifier(void)
-> register/unregister notifier is added for above notifier callback.

To avoid "LOG_TAG" redefinition issue, header file including order is changed.

Change-Id: I7118e64c9ed7eb8cd7cff6ce0e12a7873e477805
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>