msm_fb : Avoid wait for vsync when Timing Generator is off

Add checks for Timing Generator enable status before waiting
for vsync from it. This will avoid race conditions during
suspend-resume scenarios.

CRs-fixed: 406752

Signed-off-by: Pradeep Jilagam <pjilagam@codeaurora.org>
(cherry picked from commit 282a601efdb0a74b5f65a881f4e4b638d0ef9cee)

Change-Id: I9c0c2bf5e48f59c69c6928c15779b172e9ed2dc1
Signed-off-by: Gopal Goberu <ggober@codeaurora.org>