leds: qpnp-leds: Modularize regulator operation for LED

Previous implementation does not give a concise logic and is
not easy to operate regulators for LED when other LEDs are in
need. By modularizing regulator operation, it is easy to use the
logic when other LEDs need to use regulators.

Change-Id: I633c7e7d5af804c54adea6d8ac8f6567bdee2cc2
Signed-off-by: Chun Zhang <chunz@codeaurora.org>