drm-backend: limit reset/restart to output of a failed commit

When a commit fails, then only the outputs that where part of this commit should
be reset or restarted.

Otherwise an unrelated output that has another successful pending commit may be
restarted incorrectly. Then the output is in an inconsistent state and will
trigger an assertion:

weston: ../libweston/backend-drm/state-propose.c:627: drm_output_propose_state: Assertion `!output->state_last' failed.

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