drm/mxm: fix Kconfig options for when to build MXM WMI driver.

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/platform/x86/Kconfig b/drivers/platform/x86/Kconfig
index 9491457..006c6d5 100644
--- a/drivers/platform/x86/Kconfig
+++ b/drivers/platform/x86/Kconfig
@@ -755,7 +755,7 @@
 
 config MXM_WMI
        tristate "WMI support for MXM Laptop Graphics"
-       depends on WMI
+       depends on ACPI_WMI
        ---help---
           MXM is a standard for laptop graphics cards, the WMI interface
 	  is required for switchable nvidia graphics machines