drm/fb-helper: Remove unused inline function drm_fb_helper_defio_init()

Since commit 8e86dee0 ("drm/fb-helper: Remove drm_fb_helper_defio_init() and update docs")
this inline helper not used anymore.

Fixes: 8e86dee0

 ("drm/fb-helper: Remove drm_fb_helper_defio_init() and update docs")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230725021317.8080-1-yuehaibing@huawei.com