LinuxKPI: Allow lkpi_iic driver to be a child of drm device.

i915kms exposes "AUX #/port #" I2C adapters as drm children

MFC after: 1 week
Reviewed by: hselasky, manu
Differential Revision: https://reviews.freebsd.org/D35046

(cherry picked from commit f49cddbda67d6f49f1794c93270d3d923140d620)