backend-drm: handle commit failures correctly

Currently only the return value of the last backend that implements
repaint_flush() is actually used. That only works because the drm backend is the
only backend that implements repaint_flush().
In the drm backend only the return value of the last device is used. And if a
failure is actually detected, then all repainted outputs of all backends are
reset. This can trigger asserts:
weston_output_schedule_repaint_reset() (or _restart()) will change the state of
the output but the backend that did not cause the failure will still call
weston_output_finish_frame().
The same thing can happen with multiple devices in the drm backend. Or outputs
get stuck if an error is dropped.

Since the drm backend is the only one that implements repaint_flush() anyways,
move the failure handling into the backend and make it device specific.
This way only the outputs that need it are reset.

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