draw: asst. clean-up in draw_context.h

Signed-off-by: Brian Paul <brianp@vmware.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17064>