display: Move plugin struct display_plugin to core

display: Move plugin struct display_plugin to core

The struct display_plugin has been relocated:
 - src/display/plugin-common/display-ops.h
   => src/display/display-plugin.h

Added wrapper for each member variable/function of the structure.
Those wrapper access/invoke matching plugin variable/function if exist.
 - display_plugin_handle()
 - display_plugin_pm_lock_internal()
 - display_plugin_pm_unlock_internal()
 - display_plugin_pm_change_internal()
 - display_plugin_update_pm_setting()
 - display_plugin_get_lock_screen_state()
 - display_plugin_get_system_wakeup_flag()
 - display_plugin_set_system_wakeup_flag()
 - display_plugin_device_flags_to_string()
 - display_plugin_auto_brightness_control()

The structure's member would grow as more plugin code will move to
the display core. Later, this struct display_plugin and wrapper can
be replaced by plugin-api-deviced.

Change-Id: I2ebc6b77b647764cbc1d4ceb55d17fdc2fee6af1
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>