drm/fb-helper: Check fb_deferred_io_init() return value

The fb_deferred_io_init() can fail and return an errno code but currently
there is no check for its return value.

Fix that and propagate to errno to the caller in the case of a failure.

Fixes: d536540f ("drm/fb-helper: Add generic fbdev emulation .fb_probe function")
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230121192418.2814955-3-javierm@redhat.com