drm/i915/bxt: Don't reprogram an already enabled DDI PHY

If BIOS has already programmed and enabled a PHY, don't reprogram it as
that may interfere with the currently active outputs. A follow-up patch
will add state verification, so we can catch any misconfiguration on
BIOS's behalf.

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/1459515767-29228-14-git-send-email-imre.deak@intel.com