x86: move desc_empty to where they belong

This patch moves the (duplicated) desc_empty implementation to desc.h,
where the descriptor things belong.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>