panfrost: Add madvise support to BO cache

panfrost: Add madvise support to BO cache

The kernel now supports madvise ioctl to indicate which BOs can be freed
when there is memory pressure. Mark BOs purgeable when they are in the
BO cache. The BOs must also be munmapped when they are in the cache or
they cannot be purged.

We could optimize avoiding the madvise ioctl on older kernels once the
driver version bump lands, but probably not worth it given the other
driver features also being added.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>