drm/fb_cma_helper: do not free fbdev if there is none

If fbdev emulation is not in use (or not built-in), fb_helper.fbdev
is NULL. Don't call calling drm_fbdev_cma_defio_fini in this case.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161020003221.2941-2-stefan@agner.ch