drm/i915: debugfs/i915_gem_seqno_info does not need rpm nor struct_mutex

i915_gem_seqno_info() supplies its own spinlocks to access the waiters,
and doesn't need any GGTT or mmio access.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20160903065343.25151-1-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>