disp: msm: sde: fix vblank handling during trusted UI

Currently vblank enable/disable events are processed based
on the vm ownership check. Use the vblank helper functions
instead, during the transitions to avoid processing the
vblank from DRM framework level.

Change-Id: Icb2bd569e73d9206a234489a288320fecf631845
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>