drm/msm/sde: take connector reference from conn state

Update the DRM connector state callbacks to call helper
functions provided by the DRM/KMS framework wherever
appropriate. This allows updated framework semantics
to be picked up automatically by our state handling
functions.

In particular, the current set of helper functions also
take a reference count on the DRM connector to account
for the fact that the connector state structures also
contain a pointer to the base connector object.

Change-Id: Ib890672c451b9bdec644be905449085040b8c758
Signed-off-by: Clarence Ip <cip@codeaurora.org>