drm: Limit the number of .get_vblank_counter() retries

Pontential infinite loops in the vblank code are a bad idea. Add some
limits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>