drm/i915: Add debugfs knobs for VLVCHV memory latency values

Allow tweaking the VLV/CHV memory latencies thorugh sysfs, like we do
for ILK+.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>