x86/vPMU: move vpmu_ops to .init.data

x86/vPMU: move vpmu_ops to .init.data

Both vendors' code populates all hooks, so there's no need to have any
NULL checks before invoking the hook functions. With that the only
remaining uses of the object are in alternative_{,v}call(), i.e. none
after alternatives patching.

In vpmu_arch_initialise() the check gets replaced by an opt_vpmu_enabled
one, as I couldn't convince myself that the pre-existing checks would be
sufficient to cover all possible cases.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>