drm/exynos: consider no iommu support to console framebuffer

This patch considers no iommu support to kernel space mapping
of console framebuffer.

Without iommu, we get physical address instead of device address
after dma_alloc_attrs function is called. So we should consider
the case without iommu when it maps console framebuffer with
kernel space.

Changelog v2:
- calll vunmap function only with iommu support.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>