zink: unset ctx->program pointers when an unref destroys the object

if we destroy a program object which is currently the "active" program then
we need to unset the pointer to avoid invalid access

also unset injected tcs pointers where appropriate

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9470>