drm/panthor: Fix race condition when gathering fdinfo group samples

Commit e16635d8 ("drm/panthor: add DRM fdinfo support") failed to
protect access to groups with an xarray lock, which could lead to
use-after-free errors.

Fixes: e16635d8 ("drm/panthor: add DRM fdinfo support")
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250130172851.941597-6-adrian.larumbe@collabora.com
Link: https://patchwork.freedesktop.org/patch/msgid/20250107173310.88329-1-florent.tomasin@arm.com