drm/vmwgfx: Only build on X86

ioremap_cache() is currently not available on some architectures.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NThomas Hellstrom <thellstrom@vmware.com>

drivers/gpu/drm/vmwgfx/Kconfig

 config DRM_VMWGFX
 	tristate "DRM driver for VMware Virtual GPU"
-	depends on DRM && PCI
+	depends on DRM && PCI && X86
 	select FB_DEFERRED_IO
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA