ecore_drm2: make flip to NULL buffer mean something

ecore_drm2: make flip to NULL buffer mean something

When triple buffering we'll have a buffer in ecore_drm2's "next" position.
Until now we've had to query it from the engine then try to re post it.

Also, when generating ticks we need to flip to the current buffer when no
changes have been made to get another callback.

Now a NULL fb to fb_flip will either flip to next, if available, or current
if there's nothing new to flip to.