x86/hpet: Wrap legacy clockevent in hpet_channel

For HPET channel 0 there exist two clockevent structures right now:
  - the static hpet_clockevent
  - the clockevent in channel 0 storage

The goal is to use the clockevent in the channel storage, remove the static
variable and share code with the MSI implementation.

As a first step wrap the legacy clockevent into a hpet_channel struct and
convert the users.

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/20190623132436.368141247@linutronix.de