msm_fb: display: Create sysfs entries in mdp_probe for MDP4 targets

vsync sysfs entries should be created in mdp_probe instead of
creating in panel on. This avoids waiting for the first event
control in framework

Change-Id: I983731481ea89bb7666be5f2a9f75a1e04fd963f
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>