fix cursor loss w/ legacy drm and software cursor (tested on etnaviv)

I tested sway today on an etnaviv platform (imx6qp/reform) and it worked fine except that the cursor disappeared as soon as I moved it into the swaybar and would not reappear. This was because swaybar clears and then immediately sets a cursor. Both fail in DRM, but the failure of clearing is not seen, leading to a wrong state. This PR adds a missing error handling for the drmModeSetCursor() case where an empty cursor is set.