x86/db: Split out dr6/7 handling

DR6/7 should be handled before nmi_enter() is invoked and restore after
nmi_exit() to minimize the exposure.

Split it out into helper inlines and bring it into the correct order.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505135314.808628211@linutronix.de