msm: mdss: Prevent VSYNC enable when assertive display stopped
msm: mdss: Prevent VSYNC enable when assertive display stopped

Only enable vsync when vsync requested and assertive display is on.
Prevent NULL pointer dereference if vsync is called after assertive
display is disabled.

Change-Id: Ibc3eae66c2dc5869f26c145cb310502507fbfe69
Signed-off-by: Carl Vanderlip <c1ee614365181d9b29d91b6c5237384ed28bec62@codeaurora.org>

TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas