drm/i915: make pool objects read-only

For our current users we don't expect pool objects to be writable from
the gpu.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 4f7af19 ("drm/i915: Support ro ppgtt mapped cmdparser shadow buffers")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119150154.18249-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gt/intel_engine_pool.c

@@ -104,6 +104,8 @@ node_create(struct intel_engine_pool *pool, size_t sz)
 		return ERR_CAST(obj);
 	}
 
+	i915_gem_object_set_readonly(obj);
+
 	node->obj = obj;
 	return node;
 }