drm: atmel-hlcdc: fix vblank initial state

drm_vblank_on() now warns on nested use or if vblank is not properly
initialized. This patch fixes Atmel HLCDC vblank initial state.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Sylvain Rochet <sylvain.rochet@finsecur.com>