drm/i915: Rename SNB/IVB CPU eDP signal level funcs

To make the intent more clear, let's rename the signal level funcs for
the SNB/IVB CPU eDP.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517170309.28630-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>