disp: msm: dp: fix locking during link maintenance

Ensure that session lock is held while handling link maintenance
to ensure that connect/disconnect handling and link maintenance are
serialized. This will avoid un-intended interleaving of session
setup/teardown with link maintenance which can result in link training
failures.

CRs-Fixed: 2565704
Change-Id: I96bb5b01a80433e26d675a019b8827de9fecb75b
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>