v3d: do not use kmsro to create screens on rpi4
What does this MR do and why?
In commit 8f6211 we switched to use kmsro to create the screen display with v3d, so you could have MESA_LOADER_DRIVER_OVERRIDE=v3d
in rpi4 and would work. But this situation is a bit artificial, and this creates a dependency on kmsro.
So instead, don't use kmsro, and just print a warning in case forcing manually v3d in real hardware (rpi4 uses vc4 to create displays).