display: add dummy display for iot-headless

display: add dummy display for iot-headless

For who want cpulock directly via dbus API, not device API on headless
target, add dummy dbus object that receives those request not to emit
error.

Such a weird call, request for display function on headless profile,
is basically because display module provides state locking including
sleep state(cpulock), which should have been handled by power module.
Therefore in the future, moving cpulock from display to power module
should be followed by removing this dummy.

Change-Id: If8f0b449f65832ddf705b9e7ad4e461c20f6a102
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>