drm: idiot-proof vblank

After spending slightly more time than I'd care to admit debugging the
various and presumably spectacular way things fail when you pass too low
a value to drm_vblank_init() (thanks console-lock for not letting me see
the carnage!), I decided it might be a good idea to add some sanity
checking.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>