drm/i915: Drop drm_vblank_cleanup

On the load error path we can't have pending vblank interrupts, and on
unload we already call drm_atomic_helper_shutdown beforehand! So all
good to nuke it.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-5-daniel.vetter@ffwll.ch