drm: simplify drm_sysfs_destroy() via IS_ERR_OR_NULL()

Simplify `foo == NULL || IS_ERR(foo)` via IS_ERR_OR_NULL(). This is
pretty commonly used all over the kernel, especially for debugfs/sysfs
cleanup paths.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>