i965/bufmgr: fix invalid assertion

The idea behind this assert is that if a buffer is in bufmgr->handle_table it's because it has been shared from i965 to the outside. This is when we add the drm FD associated to this BO to bo->exports.

But we also import buffer from the outside into i965 and those buffers don't have an associated drm FD added to bo->exports.

If you import the same buffer more than once, you'll run into this assert.

Signed-off-by: Lionel Landwerlin lionel.g.landwerlin@intel.com