drm/fbdev-generic: Inline clean-up helpers into drm_fbdev_fb_destroy()

The fbdev framebuffer cleanup in drm_fbdev_fb_destroy() calls
drm_fbdev_release() and drm_fbdev_cleanup(). Inline both into the
caller. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125200415.14123-10-tzimmermann@suse.de