asahi: Flip kmsro around to allocate on the GPU

asahi: Flip kmsro around to allocate on the GPU

Our display controller can handle arbitrary GPU imports, so there is no
reason to use dumb KMS buffers. Allocate everything on the GPU instead.

This also allows us to be lazy about mapping things to the KMS side, so
only clients that really want a KMS handle actually do that, which stops
us from ending up with a bunch of junk mapped to DCP (e.g. X11 clients
always request SCANOUT even under XWayland).

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22353>