drm/exynos: consider buffer allocation without iommu

This patch fixes the issue that when buffer allocation is requested
without iommu, the allocation is failed.

Without iommu, dma_alloc_attrs function allocates some memory region
and returns cpu address so this patch makes the cpu address to be set
to buf->kvaddr correctly.

Changelog v2:
- fix buffer free
  . Without iommu, dma_free_attrs function requires kernel space address
    as argument. So it changes the argument, buf->pages to buf->kvaddr.

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