drm/i915/dp: move edp vdd enable/disable at a lower level in i2c-over-aux

This is prep work for conversion to generic drm i2c-over-aux helpers
where we won't have the function to do this at.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>