drm/fb-helper: Remove drm_fb_helper_unprepare() from drm_fb_helper_fini()

Move drm_fb_helper_unprepare() from drm_fb_helper_fini() into the
calling fbdev implementation. Avoids a possible stale mutex with
generic fbdev code.

As indicated by its name, drm_fb_helper_prepare() prepares struct
drm_fb_helper before setting up the fbdev support with a call to
drm_fb_helper_init(). In legacy fbdev emulation, this happens next
to each other. If successful, drm_fb_helper_fini() later tear down
the fbdev device and also unprepare via drm_fb_helper_unprepare().

Generic fbdev emulation prepares struct drm_fb_helper immediately
after allocating the instance. It only calls drm_fb_helper_init()
as part of processing a hotplug event. If the hotplug-handling fails,
it runs drm_fb_helper_fini(). This unprepares the fb-helper instance
and the next hotplug event runs on stale data.

Solve this by moving drm_fb_helper_unprepare() from drm_fb_helper_fini()
into the fbdev implementations. Call it right before freeing the
fb-helper instance.

Fixes: 643231b2 ("drm/fbdev-generic: Minimize hotplug error handling")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230216140620.17699-1-tzimmermann@suse.de