drm: add private data member to drm_connector

Add a private data member to drm_connector. This is needed
by some driver to keep its own driver specific data. For
example, when connector and bridge are created in different
modules, and the bridge driver will overwrite some ops of
the connector, so the these overwritten connector ops have
to access the bridge driver specific data in order for the
device specific implementation. In this case the private
member could be utilized.

CRs-Fixed: 2140653
Change-Id: Ib1f1d1c43a67ee4390a7f0451c315fd075601741
Signed-off-by: Ray Zhang <rayz@codeaurora.org>