WMI: initialize wmi_blocks.list even if ACPI is disabled

Even if we don't want to register the WMI driver, we should initialize
the wmi_blocks list to be empty, since we don't want the wmi helper
functions to oops just because that basic list has not even been set up.

With this, "find_guid()" will happily return "not found" rather than
oopsing all over the place, and the callers will then just automatically
return false or AE_NOT_FOUND as appropriate.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>