drm/fb-helper: fix leaks in error path of drm_fb_helper_fbdev_setup

drm/fb-helper: fix leaks in error path of drm_fb_helper_fbdev_setup

After drm_fb_helper_fbdev_setup calls drm_fb_helper_init,
"dev->fb_helper" will be initialized (and thus drm_fb_helper_fini will
have some effect). After that, drm_fb_helper_initial_config is called
which may call the "fb_probe" driver callback.

This driver callback may call drm_fb_helper_defio_init (as is done by
drm_fb_helper_generic_probe) or set a framebuffer (as is done by bochs)
as documented. These are normally cleaned up on exit by
drm_fb_helper_fbdev_teardown which also calls drm_fb_helper_fini.

If an error occurs after "fb_probe", but before setup is complete, then
calling just drm_fb_helper_fini will leak resources. This was triggered
by df2052cc922 ("bochs: convert to drm_fb_helper_fbdev_setup/teardown"):

    [   50.008030] bochsdrmfb: enable CONFIG_FB_LITTLE_ENDIAN to support this framebuffer
    [   50.009436] bochs-drm 0000:00:02.0: [drm:drm_fb_helper_fbdev_setup] *ERROR* fbdev: Failed to set configuration (ret=-38)
    [   50.011456] [drm] Initialized bochs-drm 1.0.0 20130925 for 0000:00:02.0 on minor 2
    [   50.013604] WARNING: CPU: 1 PID: 1 at drivers/gpu/drm/drm_mode_config.c:477 drm_mode_config_cleanup+0x280/0x2a0
    [   50.016175] CPU: 1 PID: 1 Comm: swapper/0 Tainted: G                T 4.20.0-rc7 #1
    [   50.017732] EIP: drm_mode_config_cleanup+0x280/0x2a0
    ...
    [   50.023155] Call Trace:
    [   50.023155]  ? bochs_kms_fini+0x1e/0x30
    [   50.023155]  ? bochs_unload+0x18/0x40

This can be reproduced with QEMU and CONFIG_FB_LITTLE_ENDIAN=n.

Link: https://lkml.kernel.org/r/20181221083226.GI23332@shao2-debian
Link: https://lkml.kernel.org/r/20181223004315.GA11455@al
Fixes: 8741216396b2 ("drm/fb-helper: Add drm_fb_helper_fbdev_setup/teardown()")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181223005507.28328-1-peter@lekensteyn.nl