nv50,nvc0: buffer resources can be bound as other things down the line

nv50,nvc0: buffer resources can be bound as other things down the line

res->bind is not an indicator of how the resource is currently bound.
buffers can be rebound across different binding points without changing
underlying storage.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.4 10.3" <mesa-stable@lists.freedesktop.org>