display: Add getter for lock count and signal on count changed

display: Add getter for lock count and signal on count changed

The dbus method "PmlockGetLockCount" has been added to get lock count.
And the dbus signal "PowerLockCountChanged" has also been added for
broadcasting on every display lock/unlock. The existing method
"PmlockGetLockState" and signal "PowerLockCountChanged" only say in
'locked' or 'unlocked', however, the "PmLockGetLockCount" and
"PowerLockCountChanged" respond in number.

Change-Id: I63a0eb7a12c76569d4c2de7bfe9089d5f98eac0e
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>