msm: mdss: Fix race condition for vsync irq enable/disable

Disable and enable of vsync happens in separate thread, which
may lead to a race condition where enable/disable may
happen at the same time and resulting irq in disable state.
Add lock to properly synchronize the enabling/disabling
of vsync interrupts to fix race condition.

Change-Id: Ib52bea5ca7a3612c866a4a962f09725a77e40563
Signed-off-by: Pawan Kumar <pavaku@codeaurora.org>