panfrost: Remove support for legacy kernels

panfrost: Remove support for legacy kernels

Previously, there was minimal support for interoperating with legacy
kernels (reusing kernel modules originally designed for proprietary
legacy userspaces, rather than for upstream-friendly free software
stacks). Now that the Panfrost kernel is stabilising, this commit drops
the legacy code path.

Panfrost users need to use a modern, mainline kernel supporting the
Panfrost kernel driver from this commit forward.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>