drm/i915/ehl: Port C's hotplug interrupt is associated with TC1 bits

On platforms that have the MCC PCH, Port C's hotplug interrupt
bits are mapped to TC1 bits.

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011002618.3087-1-vivek.kasireddy@intel.com