drm/i915: Rename the remaining gen4 references to g4x in the DP code

i965 does not have native DP. Let's rename the remaining gen4 references
in the DP code to g4x.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517170309.28630-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>