drm/panfrost: Internally label some BOs

Perfcnt samples buffer is not exposed to UM, but we would like to keep
a tag on it as a potential debug aid.

PRIME imported GEM buffers are UM exposed, but since the usual Panfrost
UM driver code path is not followed in their creation, they might remain
unlabelled for their entire lifetime, so a generic tag was deemed
preferable. The tag is assigned before a UM handle is created so it
doesn't contradict the logic about labelling internal BOs.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250520174634.353267-3-adrian.larumbe@collabora.com