drm: Nerf the preclose callback for modern drivers

With all drivers converted there's only legacy dri1 drivers using it.
Not going to touch those, instead just hide it like we've done with
other dri1 driver hooks like firstopen.

In all this I didn't find any real reason why we'd needed 2 hooks, and
having symmetry between open and close just appeases my OCD better.
Yeah, someone else could do an s/postclose/close/, but that's for
someone who understands cocci. And maybe after this series is reviewed
and landed, to avoid patch-regen churn.

v2: s/last/post/close in the kernel-doc (Sean).

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170508082633.4214-4-daniel.vetter@ffwll.ch