drm/gem: Document locking rule of vmap and evict callbacks

drm/gem: Document locking rule of vmap and evict callbacks

The vmap/vunmap/evict GEM callbacks are always invoked with a held GEM's
reservation lock. Document this locking rule for clarity.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.d>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250322212608.40511-4-dmitry.osipenko@collabora.com