v3dv: fix crash when compositor doesn't expose wl_drm
wlroots is getting ready to drop wl_drm, since it is a legacy interface 1. However v3dv would crash in that case: wl_drm_destroy() expects a non-NULL argument.
Ideally v3dv would fallback to a non-scanout alloc when wl_drm is unavailable, but let's start with not crashing for now.