drm/i915: Remove obsolete min/max freq setters from debugfs

A more complete, and more importantly stable, interface for controlling
the RPS frequency range is available in sysfs, obsoleting the unstable
debugfs.

It's presence seems to trick people into using it, forgetting it is not
ABI.

References: https://bugs.freedesktop.org/show_bug.cgi?id=106237
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180425142334.27113-1-chris@chris-wilson.co.uk