drm/msm/dp: validate power/aux submodule pointers in unbind path

Validate the power/aux submodule pointers before calling the
respective cleanup functions since these submodules might not
have been initialized successfully during boot time.

Change-Id: I195ef84beff090eeb5b6e996883fdd503406b2db
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>