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.

v3:
- clarify commit message, add code comment (imre)

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com> (v2)
Cc: M, Satheeshakrishna <satheeshakrishna.m@intel.com>
Cc: Lespiau, Damien <damien.lespiau@intel.com>
Cc: Shankar, Uma <uma.shankar@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>