platform/x86: wmi: Incorporate acpi_install_notify_handler

As a platform driver, acpi_driver.notify will not be available,
so use acpi_install_notify_handler as we will be converting to a
platform driver.

This gives event drivers a simple way to handle events. It
also seems closer to what the Windows docs suggest that Windows
does: it sounds like, in Windows, the mapper is responsible for
called _WED before dispatching to the subdriver.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
[dvhart: merge two development commits and update commit message]
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Mario Limonciello <mario_limonciello@dell.com>
Cc: Pali Rohár <pali.rohar@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: platform-driver-x86@vger.kernel.org
Cc: linux-acpi@vger.kernel.org
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>