display: change sleep reason to DISPLAY_OFF_TIMEOUT

display: change sleep reason to DISPLAY_OFF_TIMEOUT

Technically, the sleep request of display plugin is triggered by timer,
not by display off itself.
  1. Display off, display state is changed to S_LCDOFF.
  2. Start timer, 300ms.
  3. On expiring the timer, display state is changed to S_SLEEP.
  4. The display plugin requests power module to change its state to
     DEVICED_POWER_STATE_SLEEP.
Therefore, it is proper to use enum DISPLAY_OFF_TIMEOUT instead of
just DISPLAY_OFF.

Change-Id: I93e6d092ebcc23e09db61f65d8f0b7e4dc5aac7e
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>