drm/rockchip: Refactor IOMMU initialisation

Defer the IOMMU domain setup until after successfully binding
components, so we can figure out IOMMU support directly from the VOP
devices themselves, rather than manually inferring it from the DT (which
also fails to account for whether the IOMMU driver is actually loaded).
Although this is somewhat of a logical cleanup, the main motivation is
to prepare for a change in the iommu_domain_alloc() interface.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/94eee7ab434fe11eb0787f691e9f1ab03a2e91be.1649168685.git.robin.murphy@arm.com