drm/exynos: Deal with g2d buffer info more efficiently

This patch adds g2d_buf_info structure and buffer relevant
variables moves into the g2d_buf_info to manage g2d buffer
information more efficiently.

Changelog v2:
- Fix merge conflict.

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>