power: pm8921-charger: call power_supply_changed in DCIN_VALID isr

When there is no external charger driver is registered the registered
power supplies will not be updated even though a valid DC connection
may be present on DC_IN.

Fix this by calling power_supply_changed in the DCIN_VALID interrupt
handler if no external charger is registered.

Change-Id: Icdb0a28b5f9456135c60badf726c367b02f6d762
Signed-off-by: David Keitel <dkeitel@codeaurora.org>