drm/panfrost: Add support for GPU heap allocations

The midgard/bifrost GPUs need to allocate GPU heap memory which is
allocated on GPU page faults and not pinned in memory. The vendor driver
calls this functionality GROW_ON_GPF.

This implementation assumes that BOs allocated with the
PANFROST_BO_NOEXEC flag are never mmapped or exported. Both of those may
actually work, but I'm unsure if there's some interaction there. It
would cause the whole object to be pinned in memory which would defeat
the point of this.

On faults, we map in 2MB at a time in order to utilize huge pages (if
enabled). Currently, once we've mapped pages in, they are only unmapped
if the BO is freed. Once we add shrinker support, we can unmap pages
with the shrinker.

Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808222200.13176-9-robh@kernel.org