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: NYoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: NInki Dae <inki.dae@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>