staging: vboxvideo: Use CONFIG_DRM_KMS_FB_HELPER to check for fbdefio availability

Checking for CONFIG_FB_DEFERRED_IO is not the right thing to do since
the fbdev code may be built as a module while vboxvideo is builtin.

This can only happen when DRM_KMS_FB_HELPER is not set:
a) DRM_VBOXVIDEO selects DRM_KMS_HELPER
b) DRM_KMS_FB_HELPER depends on DRM_KMS_HELPER
c) DRM_KMS_FB_HELPER selects FB_DEFERRED_IO

So when DRM_KMS_FB_HELPER is set and vboxvideo is builtin then
FB_DEFERRED_IO must be builtin too.

This commit moves the #ifdefs around fbdefio code over to
CONFIG_DRM_KMS_FB_HELPER, as other drm drivers already do, fixing:

drivers/staging/vboxvideo/vbox_fb.o: In function `vbox_fbdev_fini':
vbox_fb.c:(.text+0x36): undefined reference to `fb_deferred_io_cleanup'
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vboxvideo/vbox_fb.c

@@ -45,7 +45,7 @@
 #include "vbox_drv.h"
 #include "vboxvideo.h"
 
-#ifdef CONFIG_FB_DEFERRED_IO
+#ifdef CONFIG_DRM_KMS_FB_HELPER
 static struct fb_deferred_io vbox_defio = {
 	.delay = HZ / 30,
 	.deferred_io = drm_fb_helper_deferred_io,
@@ -173,7 +173,7 @@ static int vboxfb_create(struct drm_fb_helper *helper,
 	info->screen_base = bo->kmap.virtual;
 	info->screen_size = size;
 
-#ifdef CONFIG_FB_DEFERRED_IO
+#ifdef CONFIG_DRM_KMS_FB_HELPER
 	info->fbdefio = &vbox_defio;
 	fb_deferred_io_init(info);
 #endif
@@ -195,7 +195,7 @@ void vbox_fbdev_fini(struct drm_device *dev)
 	struct vbox_fbdev *fbdev = vbox->fbdev;
 	struct vbox_framebuffer *afb = &fbdev->afb;
 
-#ifdef CONFIG_FB_DEFERRED_IO
+#ifdef CONFIG_DRM_KMS_FB_HELPER
 	if (fbdev->helper.fbdev && fbdev->helper.fbdev->fbdefio)
 		fb_deferred_io_cleanup(fbdev->helper.fbdev);
 #endif