osd: fix hotplug hdmi causes system crash [1/1]

osd: fix hotplug hdmi causes system crash [1/1]

PD#SWPL-7009

Problem:
hotplug hdmi causes system crash
when unplug hdmi display mode is null, hwc hasn't received
event and post frame normal but osd vsync lost, which caused
wait hw vsync timeout(1s), and fd leak.

Solution:
when process every frame, if display mode is null, exit frame process
just process fence.

Verify:
verified on Raven, franklin

Change-Id: Icbe7cd76776e46a6ed79c6cee3ea05772cc5f9d2
Signed-off-by: Pengcheng Chen <pengcheng.chen@amlogic.com>