display: skip state change operation if next state is same as current state

display: skip state change operation if next state is same as current state

- Previously, display state was set again even if next state
  is same as current state. Thus display was turned on for a while
  during turning off the display since pm_cur_state was updated
  after turning off the display and checking if the state is transitable.

- Now, display transition is ignored if next state is same as
  current state. Thus the state is not transitable in the case.

Change-Id: I605916c5e2235e46b0b8b04375b4427b32584665
Signed-off-by: taeyoung <ty317.kim@samsung.com>