drm/exynos: Move dma_addr out of exynos_drm_fb

This can be calculated from the GEM BO DMA address as well as the offset
stored in the base framebuffer.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>