drm/sun4i: call drm_vblank_init with correct number of crtcs

If we want to have vblank on both pipelines at the same time, we need
to call drm_vblank_init with num_crtcs = 2.

Instead, since the crtc init calls correctly set mode_config.num_crtc,
we can move the drm_vblank_init call to after the crtc init code is
called, which is the component bind part. Then we can just pass
mode_config.num_crtc in.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908075016.18657-8-wens@csie.org