wl2_surface_dmabuf: improve surface flush

wl2_surface_dmabuf: improve surface flush

We used to abandon all buffers even if they were locked. This can't
actually free them until they're all released by the compositor.

Instead just free any buffer the compositor doesn't have locked, so we can
still re-use them when they're released without needing a full redraw.

There's probably room for additional cleverness here.  If we get a new
frame event before the buffer release we may want to keep it, and if we
get the release first we may want to abandon it.