swr: make sure that all rendering is finished on shader destroy

swr: make sure that all rendering is finished on shader destroy

Rendering could still be ongoing (or have yet to start) when the shader
is deleted. There's no refcounting on the shader text, so insert a
pipeline stall unconditionally when this happens.

[Note, we should instead introduce a way to attach work to
fences, so that the freeing can be done in the current fence.]

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>