drm/i915: Rename s/i9xx/g4x/ in DP code

g4x is the first platform with DP support, so let's name the relevant
functions as g4x_ instead i9xx_ to avoid confusion.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456932138-14004-3-git-send-email-ville.syrjala@linux.intel.com