Adopt a dpms cache

Adopt a dpms cache

In the previos structure, deviced sets dpms through Enlightenment,
whereas gets dpms by reading the lcd power node directly. There is
a possibility of a miasmatch between the actual dpms state and the
lcd power node. The cached dpms value is alternatively used for
this situation in replacement of the get_lcd_power().

Change-Id: Id0144cd76cb017694198856f6b40ff7162b61e64
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>
Signed-off-by: Hyotaek Shim <hyotaek.shim@samsung.com>
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>