drm/i915: Move VLV/CHV prepare_pll later

With DPIO powergating active on CHV, we can't even access the DPIO PLL
registers until the lane power state overrides have been enabled. That
will happen from the encoder .pre_pll_enable() hook, so move
chv_prepare_pll() to happen after that point, which puts it just before
chv_enable_pll() actually.

Do the same for VLV to avoid accumulating weird differences between the
platforms. Both platforms seem happy with the new arrangement.

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