Fix cursors in DRM when no cursor plane is available
Prior to this we allocated a fake plane, which made subsequent operations on the plane fail. Now we just fail, and wlroots will fallback to a software cursor.
Test plan: line 504 of backend/drm/drm.c
, add crtc->cursor = NULL;
. Or test with a card without cursor planes (cc @ascent12).
Fixes #461 (closed)