Avoids a little duplication (matching two variants, instead of three
backends).
The behavior, including errors and panics, should be unchanged for now.
Performance should also not be impacted.
This should help for adding llvmpipe rendering without a device node on
the kms backend, or for adding a variant for pixman.
This is increasingly not just related to screencopy, so it's weird to
add there. I don't see any other module that fits, so add one called
"quirks" (like the Linux kernel uses for device-specific handling in
generic drives).
