drm/i915: add i915_ips_status debugfs entry

It just prints whether it's supported/enabled/disabled. Feature
requested by the power management team.

v2: Checkpatch started complaining about seq_printf with 1 argument.

Requested-by: Kristen Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>