drm/v3d: Refcount v3d_stats

drm/v3d: Refcount v3d_stats

Convert `v3d_stats` from embedded structs to heap-allocated, refcounted
objects. This decouples the stats lifetime from the containing
structures (this is, `v3d_queue_state` and `v3d_file_priv`), allowing
jobs to safely hold their own references to stats objects even after the
file descriptor is closed.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patch.msgid.link/20260306-v3d-reset-locking-improv-v3-3-49864fe00692@igalia.com
Co-developed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>