timers: Split out get next timer interrupt

Split out get_next_timer_interrupt() to be able to extend it and make it
reusable for other call sites.

No functional change.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240221090548.36600-3-anna-maria@linutronix.de