'drm_vblank_init()' can fail. So handle this (unlikely) error.

Fixes: bbbe775e ("drm: Add support for Amlogic Meson Graphic Controller")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6cbf3d70ac3904489c7194c895225c4103aebb96.1520885192.git.christophe.jaillet@wanadoo.fr