drm/msm/hdmi: remove useless kref

A left-over from prior to component framework.  The original intent was
to deal with hdmi getting unloaded before the master component, but that
isn't really going to work anyways.  These days with the component
framework taking care to unload the master component first, we don't
have to worry about this.

Signed-off-by: Rob Clark <robdclark@gmail.com>