drm: unexport function drm_vblank_no_hw_counter()

The function drm_vblank_no_hw_counter() is now only used in core vblank
wrapper code.  Let's unexport it by making it a static function.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1486458995-31018-4-git-send-email-shawnguo@kernel.org