drm/i915/cnl: Kill _MMIO_PORT6 macro

This patch replaces use of remaining _MMIO_PORT6 macro and removes the
macro.

Changes Since V1:
 - Rebase

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180314080653.9444-3-mahesh1.kumar@intel.com