x86: make sure we really have an hpet mapping before using it

Impact: prepare the hpet code for Xen dom0 booting

When booting in Xen dom0, the hpet isn't really accessible, so make
sure the mapping is non-NULL before use.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>