drm/exynos: move dma_addr attribute from exynos plane to exynos fb

DMA address is a framebuffer attribute and the right place for it is
exynos_drm_framebuffer not exynos_drm_plane. This patch also introduces
helper function for getting dma address of the given framebuffer.

Changelog v2:
- use state->fb instead of plane->base.fb.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>