drm: make drm_vblank_count_and_time() static

As they are not used anywhere outside drm_irq.c make them static.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1467677092-5089-1-git-send-email-gustavo@padovan.org