drm/i915: Handle SDEISR according to PCH rather than platform

The South Display is part of the PCH so we should technically be basing
our port detection logic off the PCH in use rather than the platform
generation.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127221314.575575-1-matthew.d.roper@intel.com