panfrost: Probe G31/G52 if PAN_MESA_DEBUG=bifrost

panfrost: Probe G31/G52 if PAN_MESA_DEBUG=bifrost

We're not *quite* ready to open the flood gates on Bifrost (a major
blocker is CI, which is itself blocked on the lockdowns - expected to be
resolved in the coming months..)

Nevertheless, let's add a debug option to probe on compatible Bifrost
devices to avoid keeping out-of-tree patches around.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5272>