drm/exynos: Clean up some G2D codes for readability

This patch just cleans up G2D codes for readability.

For this, it changes the member of g2d_cmdlist_node, obj_type into
buf_type.

Changelog v2:
- Revert irrelevant codes.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>