The lastclose callback in struct drm_driver is unused. Remove it. Also
update documentation.

v2:
- update to use drm_lastclose()
- fix typo in documentation
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240812083000.337744-9-tzimmermann@suse.de