drm/i915: Make the ring IMR handling private

As the IMR for the USER interrupts are not modified elsewhere, we can
separate the spinlock used for these from that of hpd and pipestats.
Those two IMR are manipulated under an IRQ and so need heavier locking.

Reported-and-tested-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>