backend/drm: always create multigpu renderers for primary device

Multi-gpu code needs a context for each GPU in the system, but as of now we only create renderers for secondary devices. This always creates the multigpu renderer so we can use it for copying.

This is useful in !4055 and a hardware cursor fix I haven't yet posted which use the DRM mgpu_renderer to perform copies.

I don't think there would be any issues merging this on its own, but given that it's a prerequisite for the above maybe it would want to wait to go in until one of those is approved? Open to suggestions but still figured this should be reviewed separately.