drm/i915: Disable RC6 before configuring in on VLV/CHV

Follow the sequence in the BIOS spec and clear the RC_CONTROL register
before changing any of the other RC6/RP registers.

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