nv50/ir: adjust overlapping logic to take fileIndex-relative offsets

nv50/ir: adjust overlapping logic to take fileIndex-relative offsets

If the fileIndex is different, that means they are in logically
different spaces. However if there's also a relative offset, then they
could end up pointing at the same spot again.

Also add a note about potential for multiple buffers to overlap even if
they're at different file indexes. However that's potentially lowered
away by the point that this logic hits.

Not known to fix any specific application or test.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>