drm/i915: Merge pre/postclose hooks

There's really not a reason afaics that we can't just clean up
everything at the end, in the terminal postclose hook: Since this is
closing a file descriptor we know no one else can have a reference or
a thread doing something with that drm_file except the close code.
Ordering shouldn't matter, as long as we don't kfree before we clean
stuff up.

In the past this was more relevant when drivers still had to track and
clean up pending drm events, but that's all done by the core now.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Liviu Dudau <Liviu.Dudau@arm.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-13-daniel.vetter@ffwll.ch