drm/i915: Unref context on failed eb_create

I opted to do this instead of grabbing the context reference after
eb_create since eb_create can potentially call the shrinker, and that
makes things very complicated. This simple patch balances the ref count
without requiring a great deal of review to make sure the shrinker path
is safe.

Theoretically (by design) the shrinker can end up destroying a context,
which enforces the reasoning for doing the fix this way instead of
moving the reference to later in the function.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1142,6 +1142,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 
 	eb = eb_create(args);
 	if (eb == NULL) {
+		i915_gem_context_unreference(ctx);
 		mutex_unlock(&dev->struct_mutex);
 		ret = -ENOMEM;
 		goto pre_mutex_err;