backend/drm: add proper error handling to wlr_drm_backend_create
backend/drm: add proper error handling to wlr_drm_backend_create

Some listeners weren't removed and caused a use-after-free with
e.g. vkms when used as a secondary GPU.

swaywm/wlroots,SirCmpwn/wlroots,swaywm/wlroots