fbdev_window: cleanups

I've used the same idea from wayland_window.cpp, having
the buffers placed in a std::list and marked busy/free via the busy
member.

Fixed the crash which apparently was caused by reference counting but
in fact was dequeueBuffer not calling incRef on the buffer.

When DEBUG_FBDEV_WINDOW is defined, enable tracing in fbdev_window.cpp

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>