drm/i915: Report the correct errno from i915_gem_context_open()

Fixup the errno as we adjusted the error path to receive the errno and
not compute it itself from ERR_PTR(ctx) anymore.

drivers/gpu/drm/i915/i915_gem_context.c:793 i915_gem_context_open() warn: passing a valid pointer to 'PTR_ERR'
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 3aa9945a ("drm/i915: Separate GEM context construction and registration to userspace")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325090413.19906-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_context.c

@@ -709,7 +709,7 @@ int i915_gem_context_open(struct drm_i915_private *i915,
 	idr_destroy(&file_priv->context_idr);
 	mutex_destroy(&file_priv->vm_idr_lock);
 	mutex_destroy(&file_priv->context_idr_lock);
-	return PTR_ERR(ctx);
+	return err;
 }
 
 void i915_gem_context_close(struct drm_file *file)