drm/i915: debugfs/i915_gem_interrupt_info does not need struct_mutex

i915_gem_interrupt_info() only looks at mmio registers, and the waiters
under a spinlock. It doesn't need struct_mutex (but does need the rpm
wakelock for mmio access). Maybe useful using get_if_notidle?

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20160903065343.25151-2-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>