drm: Make sure drm_vblank_no_hw_counter isn't abused

Shouldn't be possible since everyone kzallocs this, but better safe
than sorry. Random drive-by-idea really.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Rodrigo Vivi <rdorigo.vivi@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1470673493-14304-1-git-send-email-daniel.vetter@ffwll.ch