drm/panfrost: Add a module parameter to expose unstable ioctls

We plan to expose performance counters through 2 driver specific
ioctls until there's a solution to expose them in a generic way.
In order to be able to deprecate those ioctls when this new
infrastructure is in place we add an unsafe module parameter that
will keep those ioctls hidden unless it's set to true (which also
has the effect of tainting the kernel).

All unstable ioctl handlers should use panfrost_unstable_ioctl_check()
to check whether they're supposed to handle the request or reject it
with ENOSYS.

Suggested-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618081648.17297-3-boris.brezillon@collabora.com