drm/rockchip: Check iommu itself instead of it's parent for device_is_available

When iommu itself is disabled in dts, we should
fallback to non-iommu buffer, check iommu parent
is meanless here.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210627084737.309163-1-andy.yan@rock-chips.com