drm/exynos: add async G2D execution to libdrm (v3)

Hello,

this series exposes async execution of G2D command buffers to userspace. Also includes is a small performance analysis test, which can also be used to stress test the engine. The async operation is of course also tested.

Please review and let me know what I can improve.

v3: Rewrote handling of vendor-specific events. The series is now based on [1]. Also added two small fixes (error handling, break statement).

With best wishes, Tobias

[1] https://patchwork.kernel.org/patch/6262541/