backend-drm: improve atomic commit failure handling

When an atomic commit fails then the output will be stuck in
REPAINT_AWAITING_COMPLETION state. It is waiting for a vblank event that was
never scheduled.
If the error is EBUSY then it can be expected to be a transient error. So
propagate the error and schedule a new repaint in the core compositor.

This is necessary because there are some circumstances when the commit can fail
unexpectedly:
- With 'state_invalid == true' one commit will disable all planes. If another
  commit for a different output is triggered immediately afterwards, then this
  commit can temporarily fail with EBUSY because it tries to use the same
  planes.
- At least with i915, if one commit enables an output then a second commit for a
  different output immediately afterwards can temporarily fail with EBUSY. This
  is probably caused by some hardware interdependency.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>