disp: msm: add support for vm event register framework

Besides SDE, other subdrivers may be interested in participating in
the VM switch. This change provides framework for display dependent
drivers like DSI, DP and RSCC to register for various VM switch
event hooks.

The following hooks to provided through msm_vm_ops:
post_hw_acquire: invoked before the first frame push after gaining
                 HW access.
pre_hw_release: invoked after the last frame commit before releasing
                the HW.
check:       check with vm clients for their readiness for HW
             releasing.

Change-Id: I616db04e979f78f76f6f97ee3b068dd348339ab6
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>