drm/i915/chv: Set min freq to RPn on CHV.

With latest Punit FW, vgg input voltag drop falling to minimum is fixed.
So reverting the WA patch & moving to turbo freq opreation range to [RPn -> RP0]

This is not a 1:1 revert of the commit 5b7c91b7.
You can refer to commit 5b5929cb ("drm/i915/chv: remove
pre-production hardware workarounds") as the reason for the discrepancy

commit 5b7c91b7


Author: Deepak S <deepak.s@linux.intel.com>
Date:   Sat May 9 18:15:46 2015 +0530

    drm/i915/chv: Set min freq to efficient frequency on chv

v2: Fix inconsistent return type. (Chris)
v3: drop pre-production hw case (Ville)

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471007801-86075-1-git-send-email-deepak.s@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>