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>