[libdrm] improvements to userspace exynos component

[Tobias Jakobi]

Hello,

here are some miscellaneous improvements (small features, bugfixes, spelling fixes, etc.) for the exynos component of libdrm. The general idea is to let userspace use the G2D engine functionality more 
efficiently.

If someone is interested in an application that actually makes use of this, the RetroArch frontend has a custom video backend:
https://github.com/libretro/RetroArch/blob/master/gfx/drivers/exynos_gfx.c


Please review and let me know what I can improve.

With best wishes,
Tobias