drm/i915/bxt: don't use unsupported port detection
The port detection register flags in SFUSE_STRAP and DDI_BUF_CTL_A are
not defined for BXT, so don't use them.
Suggested by Satheesh.
v2:
- DDI_BUF_CTL_A bit 0 is not useful on BXT. Making changes to use this
bit when simulator or BXT is not applicable. Code re-arranged as per
Damien's suggestion.