iris: silence const warning

not sure why this is labeled const, I'm pretty sure we are taking the
reference and owning this, so there's no particular reason we can't
change it.  it certainly seems to be working for non-compute.  and,
freedreno's ir3_shader.c seems to do this as well.  still...gross :/

src/gallium/drivers/iris/iris_program.c

@@ -156,7 +156,7 @@ iris_create_compute_state(struct pipe_context *ctx,
 {
    assert(state->ir_type == PIPE_SHADER_IR_NIR);
 
-   return iris_create_uncompiled_shader(ctx, state->prog, NULL);
+   return iris_create_uncompiled_shader(ctx, (void *) state->prog, NULL);
 }
 
 static void