drm/i915: Register engines early to avoid type confusion

Commit 1ec23ed7126e ("drm/i915: Use uabi engines for the default engine
map") switched from using for_each_engine() to for_each_uabi_engine() to
iterate over the user engines. While this seems to be a sensible change,
it's only safe to do when the engines are actually chained using the
rb-tree structure which is not the case during early driver
initialization where it can be either a lock-less list or regular
double-linked list.

In fact, the modesetting initialization code may end up calling
default_engines() through the fb helper code while the engines list
is still llist_node-based:

i915_driver_probe() ->
intel_display_driver_probe() ->
intel_fbdev_init() ->
drm_fb_helper_init() ->
drm_client_init() ->
drm_client_open() ->
drm_file_alloc() ->
i915_driver_open() ->
i915_gem_open() ->
i915_gem_context_open() ->
i915_gem_create_context() ->
default_engines()

Using for_each_uabi_engine() in default_engines() is therefore wrong, as
it would try to interpret the llist as rb-tree, making it find no engine
at all, as the rb_left and rb_right members will still be NULL, as they
haven't been initialized yet.

To fix this type confusion register the engines earlier and at the same
time reduce the amount of code that has to deal with the intermediate
llist state.

Reported-by: sanity checks in grsecurity
Suggested-by: Tvrtko Ursulin <[e-mail]>
Fixes: 1ec23ed7126e ("drm/i915: Use uabi engines for the default engine map")
Signed-off-by: Mathias Krause <[e-mail]>
Cc: Jonathan Cavitt <[e-mail]>
Cc: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Tvrtko Ursulin <[e-mail]>
Signed-off-by: Tvrtko Ursulin <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
[tursulin: fixed commit tag typo]
(cherry picked from commit 2b562f032fc2594fb3fac22b7a2eb3c1969a7ba3)
Signed-off-by: Rodrigo Vivi <[e-mail]>

drivers/gpu/drm/i915/i915_gem.c

···
 			goto err_unlock;
 	}
 
+	/*
+	 * Register engines early to ensure the engine list is in its final
+	 * rb-tree form, lowering the amount of code that has to deal with
+	 * the intermediate llist state.
+	 */
+	intel_engines_driver_register(dev_priv);
+
 	return 0;
 
 	/*
···
 void i915_gem_driver_register(struct drm_i915_private *i915)
 {
 	i915_gem_driver_register__shrinker(i915);
-
-	intel_engines_driver_register(i915);
 }
 
 void i915_gem_driver_unregister(struct drm_i915_private *i915)