drm/i915/bxt: Use intel_encoder->hpd_pin to check live status

Using intel_encoder's hpd_pin to check the live status
because of BXT A0/A1 WA for HPD pins and hpd_pin contains the
updated pin for the corresponding port.

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>