nouveau: ACPI support depends on X86 and X86_PLATFORM_DEVICES

If I build nouveau on ia64, Kconfig warns:

warning: (DRM_NOUVEAU) selects ACPI_WMI which has unmet direct dependencies (X86 && X86_PLATFORM_DEVICES && ACPI)
warning: (DRM_NOUVEAU) selects MXM_WMI which has unmet direct dependencies (X86 && X86_PLATFORM_DEVICES && ACPI_WMI)

Make all the ACPI support depend on X86 and select
X86_PLATFORM_DEVICES.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>