x86/hpet: Carve out shareable parts of init_one_hpet_msi_clockevent()

To finally remove the static channel0/clockevent storage and to utilize the
channel 0 storage in hpet_base, it's required to run time initialize the
clockevent. The MSI clockevents already have a run time init function.

Carve out the parts which can be shared between the legacy and the MSI
implementation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Link: https://lkml.kernel.org/r/20190623132436.552451082@linutronix.de