And what if you don't have a "default" plane as such. For example, OMAP3 has one graphics plane and two video planes, and two output paths. Each of the planes can be assigned to zero or one outputs. To accomodate this, the design should allow for CRTCs without any scanout buffers.
Some TV type stuff is a bit like that - well there may be a scanout buffer but its on a protected hardware path and no part of the system except certain bits of hardware can touch it from decrypt to output connector.
Clearly a scanout buffer isn't the only way to describe what a crtc is outputting and you need a somewhat more flexible handle including one you can acquire somehow to represent other objects like capture buffers, protected planes and live video merges.
Alan