drm/i915: Rename edp vdd funcs for consistency

edp_* are now the lower level functions and intel_edp_* the higher level
ones. One should use them in pairs.

v2: Don't return void (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>