timers: Restructure get_next_timer_interrupt()

get_next_timer_interrupt() contains two parts for the next timer interrupt
calculation. Those two parts are separated by forwarding the base
clock. But the second part does not depend on the forwarded base
clock.

Therefore restructure get_next_timer_interrupt() to keep things together
which belong together.

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/20240221090548.36600-2-anna-maria@linutronix.de