drm/i915/bxt: Power down DDI PHYs separately during the per PHY uninit

The power-down step logically belongs to the individual PHY uninit
sequence so move it there. The only functional change is that we will
power down now PHY 1 separately before PHY 0 and preserve the other bits
in the register which are defined as reserved.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459515767-29228-11-git-send-email-imre.deak@intel.com