panfrost: Rework PAN_GPU_ID mechanism

panfrost: Rework PAN_GPU_ID mechanism

Originally, PAN_GPU_ID was checked in the driver itself. I added the mechanism
to run Bifrost shader-db on my Midgard laptop. There was no drm-shim support at
this point, and this was a reasonable stop gap at the time.

Nowadays, we have a competent drm-shim implementation, which wholly replaces
this use case. So PAN_GPU_ID is only useful for drm-shim. Let's pull the code
into drm-shim and get it out of the driver. This allows NDEBUG drm-shim builds
to work properly.

While we're at it, the default emulated GPU is changed from Mali-T860 to
Mali-G52. This reflects our shifting development priorities.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Suggested-by: Icecream95 <ixn@disroot.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15930>