drm/exynos: stop copying sg table

Already struct exynos_drm_gem_buf has pages of the buffer, so we don't
need to copy from sg table of the buffer to sg table of dma-buf
attachment, just can make sg table from pages of the buffer.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>