exynos/fimg2d: add g2d_config_event

This enables us to pass command buffers to the kernel which
trigger an event on the DRM fd upon completion.
The final goal is to enable asynchronous operation of the
G2D engine, similar to async page flips.

Passing the event userdata pointer through the G2D context
was chosen to not change the current API (e.g. by adding
a userdata argument to each public functions).

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>