drm: make drm_vblank_{get,put}() 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/1465308482-15104-4-git-send-email-gustavo@padovan.org