Allow the display to turn back on

Moved the display on/off code back to display_on / _off instead of
combining it with the lcd_on/off.

Before this commit, the display would work until you turned it off
(sleep).  After that, the display would not work.  Now, the display
turns on and off as normal.

Change-Id: I757dde27b77649207f2dc4d175c0a2fe168b94ca