drm/fb-helper: Remove drm_fb_helper_fbdev_{setup, teardown}()

Both functions are unused and can be removed. Drivers should use
drm_fbdev_generic_setup() instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106124727.11641-3-tzimmermann@suse.de