drm: Put legacy lastclose work into drm_legacy_dev_reinit

Except for the ->lasclose driver callback evrything in drm_lastclose()
is all legacy cruft and can be hidden. Which means another
dev->struct_mutex site disappears entirely for modern drivers!

Also while at it change the return value of drm_lastclose to void
since it will always succeed. No one checks the return value of
close() anyway, ever.

v2: Move misplaced hunk, spotted by 0day.

Cc: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-3-git-send-email-daniel.vetter@ffwll.ch