drm/msm/sde: add pre_destroy op for sde connectors

Add pre_destroy op for sde connectors. This new op provides
a way for interface drivers to release the internally allocated
resources before deallocated the sde connector object.

Change-Id: Idd630bded85e6d05f67964ad748200549857ded1
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>