backend/drm: don't clear pending cursor FB on failed commit
backend/drm: don't clear pending cursor FB on failed commit

The previous fix tried to side-step cursor->pending_fb completely.
However that messes up our buffer locking mechanism.

Instead, stop clearing the pending cursor FB on a failed commit. The
pending cursor FB will remain for the next commit.

Fixes: 6c3d080e25e5 ("backend/drm: populate cursor plane's current_fb")

SirCmpwn/wlroots,swaywm/wlroots,swaywm/wlroots