terminal/drm: provide pipe->target() callback

terminal/drm: provide pipe->target() callback

Instead of looking for available back-buffers on each operation, set it to
NULL and wait for the next frame request. It will call back into the pipe
to request the back-buffer via ->target(), where we can do the same and
look for an available backbuffer.

This simplifies the code and avoids double lookups if we run short of
buffers.