msm: timer: Setup interrupt after registering clockevent

Some bootloaders may leave a pending interrupt for the timer and
thus msm_timer_interrupt() has a check for a NULL event handler.
Unmask and register for the interrupt after registering the
clockevent so that we can get the NULL pointer check out of the
fast path.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: David Brown <davidb@codeaurora.org>