drm/ttm: warn when resv objs are mixed in a bulk_move

The BOs in a bulk move must share all the same reservation object
to make sure that we lock the whole bulk during eviction.

Actually document and enforce that with a warning.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240312105555.3065-1-christian.koenig@amd.com