panfrost: Include glue for out-of-tree legacy code

panfrost: Include glue for out-of-tree legacy code

In addition to the DRM interface in active development, for legacy
kernels Panfrost has a small, optional, out-of-tree glue repository. For
various reasons, this legacy code should not be included in Mesa proper,
but this commit allows it to coexist peacefully with upstream Panfrost.
If the nondrm repo is cloned/symlinked to the directory
`src/gallium/drivers/panfrost/nondrm`, legacy functionality will be
built. Otherwise, the driver will build normally, though a runtime error
message will be printed if a legacy kernel is detected.

This workaround is icky, but it allows a nearly-upstream Panfrost to
work on real hardware, today. Ideally, this patch will be reverted when
the Panfrost kernel module is mature and we drop legacy support.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>