drm/i915: Fix and clean BDW PCH identification

It seems in the past we have BDW with PCH not been propperly identified
and we force it to be LPT and we were warning !IS_HASWELL on propper identification.

Now that products are out there we are receiveing logs with this incorrect WARN.
And also according to local tests on all production BDW here ULT or HALO we don't
need this force anymore. So let's clean this block for real.

v2: Fix LPT_LP WARNs to avoid wrong warns on BDW_ULT (By Jani).

Reference: https://bugs.freedesktop.org/attachment.cgi?id=110972



Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Xion Zhang <xiong.y.zhang@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>