i965/drm: Delete NULL check in brw_bo_unmap().

i965/drm: Delete NULL check in brw_bo_unmap().

I accidentally moved the bo->bufmgr dereference above the NULL check
when cleaning up this code.

While passing NULL to free() is a common pattern...passing NULL to
unmap seems pretty bad.  You really ought to know whether you have
a buffer or not.  We don't want to paper over bugs like that.  So,
just drop the NULL check altogether.

CID: 1405006

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>