drm/i915: checking IS_ERR() instead of NULL

We switched from calling i915_gem_alloc_context_obj() to calling
i915_gem_alloc_object() so the error handling needs to be updated to
check for NULL instead of IS_ERR().

Fixes: 149c86e7 ('drm/i915: Allocate context objects from stolen')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>