Revert "drm/i915: Hack to tie both common lanes together on chv"

With recent hardware/firmware there don't appear to be any glitches
on the other PHY when we toggle the cmnreset for the other PHY. So
detangle the cmnlane power wells from one another and let them be
controlled independently.

This reverts commit 3dd7b974.

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>