drm: rcar-du: Fix vblank initialization

The drm_vblank_init() takes the total number of CRTCs as an argument,
but the rcar-du driver passes a bitmask of the CRTC indices. Fix it.

Fixes: 4bf8e196 ("drm: Renesas R-Car Display Unit DRM driver")
Reported-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>