tsync: do not operate tsync_mode_switch before first video toggled [1/1]

tsync: do not operate tsync_mode_switch before first video toggled [1/1]

PD#SWPL-5131

Problem:
Some stream in tunnel mode, first audio pts is large than
AV_DISCONTINUE_THREDHOLD_MAX(60s). In audio_hw it will check pcr and
apts diff, so large difference between pcr and apts will lead sync mode
from amster to vmaster(egg:SYNC-HEVC-59FPS-DDP51)

Solution:
do not operate tsync_mode_switch before first video toggled

Verify:
verify by franklin

Change-Id: Icec2de71ea8f838146444aa3ea880f76ed8e0f13
Signed-off-by: Yingwei Long <yingwei.long@amlogic.com>