msm-fb: Fix sysfs node creation in mdp_probe for MDP3 targets

-vsync_init null check not required for mdp3 target before
 creating sysfs node.
-Set vsync suspend and resume flags appropriately in mdp_on
 for mdp3.

CRs-Fixed: 393243
Change-Id: I3366df7385ca8e2ae15053c3e58332f8e32ce0cd
Signed-off-by: Pawan Kumar <pavaku@codeaurora.org>