display: Change get_lcd_power() to dpms_get_cached_state()

display: Change get_lcd_power() to dpms_get_cached_state()

To move plugin functions without plugin dependency,
get_lcd_power() should be replaced by dpms_get_cached_state().
However, touchscreen module is dependent on display module build.
Thus, this get_lcd_power() will be changed after resolving build dependency.

Change-Id: I4d58f852cf3da29fe58df4fe3f8b69d954c77959
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>