CHV has a new knob in Punit to select between some memory power savings
modes PM2 and PM5. We can allow the deeper PM5 when maxfifo mode is
enabled, so let's do so in the hopes for moar power savings.

v2: Put the thing into a separate function to avoid churn later
v3: Don't break VLV
Reviewed-by: NVijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NArun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>