drm/fbdev-shmem: Clean up deferred I/O

Call fb_deferred_io_cleanup() upon destroying the framebuffer
device. Releases the internal memory.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 150f431a ("drm/fbdev: Add fbdev-shmem")
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
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240507152329.3085-1-tzimmermann@suse.de