drm/fbdev-generic: Minimize hotplug error handling

Call drm_fb_helper_fini() in the generic-fbdev hotplug helper
to revert the effects of drm_fb_helper_init(). No full cleanup
is required.

v3:
	* fix error in commit message (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125200415.14123-8-tzimmermann@suse.de