drm/i915/ehl: Add third combo PHY offset

v2: Rename register to _EHL_COMBOPHY_C.  (Jose)

Cc: José Roberto de Souza <jose.souza@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/20190626000352.31926-3-matthew.d.roper@intel.com