drm/vmwgfx: Rename stream output target binding tracker struct

Previous name vmw_ctx_bindinfo_so is misleading because it actually
represent so target and stream output is a new resource type that needs
tracking for SM5 capable device. Also rename binding type enum and
internal functions to reflect these belongs to so targets.

Signed-off-by: Deepak Rawat <drawat.floss@gmail.com>
Reviewed-by: Thomas Hellström (VMware) <thomas_os@shipmail.org>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>