drm/panthor: Document drm_panthor_tiler_heap_destroy::handle validity constraints

Make sure the user is aware that drm_panthor_tiler_heap_destroy::handle
must be a handle previously returned by
DRM_IOCTL_PANTHOR_TILER_HEAP_CREATE.

v4:
- Add Steve's R-b

v3:
- New patch

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240502165158.1458959-6-boris.brezillon@collabora.com