drm/i915: Match debugfs interface name to new RPS naming

Let's change the i915_cur_delayinfo to i915_frequency_info to be in sync
with new RPS naming convention.

v2: Add "i915_frequency_info" as debugfs interface name (Ben)

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>