drm/i915/bxt: Fix eDP panel fitting (v2)

drm/i915/bxt: Fix eDP panel fitting (v2)

BXT CRTC scaling uses the same gen9 codepaths as SKL; these codepaths
store panel fitter information in pipe_config->pch_pfit.  However since
HAS_PCH_SPLIT() is false for BXT we never actually wind up filling in
this structure (we wind up filling in pipe_config->gmch_pfit instead,
which is ignored when we go to program the hardware).  Make sure we
always take the PCH code path on gen9+ platforms.

v2: Use HAS_GMCH_DISPLAY() to more cleanly describe the platforms that
    actually want to use GMCH-style panel fitting.  (Ville)

Cc: Imre Deak <imre.deak@intel.com>
Cc: Chandra Konduru <chandra.konduru@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446656727-3516-1-git-send-email-matthew.d.roper@intel.com