drm/vblank: _ioctl posfix for ioctl handler

I alwasy get confused about drm_wait_vblank for a split second until I
realize it's the ioctl handler. Unconfuse me, and do it for the legacy
modeset vblank control ioctl too.

While at it also noticed that I misplaced the irq ioctl handler in the
internal header file.

Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-12-daniel.vetter@ffwll.ch