drm/rcar-du: Don't set struct drm_device.irq_enabled

The field drm_device.irq_enabled is only used by legacy drivers
with userspace modesetting. Don't set it in rcar-du.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210625082222.3845-17-tzimmermann@suse.de