drm/i915/ehl: Don't forget to handle port C's hotplug interrupts

We're mostly re-using ICL's interrupt handling on EHL, but we still need
to remember to account for the extra combo port that EHL has.  Use TGP's
mask (which includes combo port C) rather than ICP's mask when
appropriate.  Let's also skip reading TC-specific registers on this
platform since EHL doesn't have any TC ports.

v2: Base setup of SHOTPLUG_CTL_TC on whether the tc pin mask is non-zero
    rather than performing another PCH type check.  (Jose)

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
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>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730220553.15300-1-matthew.d.roper@intel.com