drm/exynos: fix layering violation of address

There is no guarantee that DMA addresses are the same as physical
addresses, but dma_to_pfn() knows how to convert a dma_addr_t to a PFN
which can then be converted to a struct page.

Suggested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>