drm/sun4i: Check return value of drm_vblank_init

drm_vblank_init can fail due to insufficient memory. Ignoring the error
and proceeding may cause the kernel to dereference an invalid pointer
when vblank is enabled.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>