zink: destroy gfx program when a shader is freed

zink: destroy gfx program when a shader is freed

there's no sense in having these objects sitting around when they can
never be used again

requires adding a zink_context* pointer to each program in order to prune
the hash table entry

Reviewed-by: Antonio Caggiano <antonio.caggiano@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5887>