drm/i915: Update PMINTRMSK on VLV/CHV after sysfs min/max freq change

Currently we don't call valleyview_set_rps() when changing the min/max
limits through sysfs if the current frequency is still within the new
limits. However that means we sometimes forget to update PMINTRMSK.
Eg. if the current frequency is at the old minimum, and then we reduce
the minum further we should then enable the 'down' interrupts in PMINTRMSK
but currently we don't.

Fix it up by always calling valleyview_set_rps() (just like we do for
!vlv/chv platforms). This also allows the code to be simplified a bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>