mm-video: vdec: Polling video driver earlier

Polling on video driver should be started immediately after
opening the device to avoid missing any events from driver.
An issue was observed where some of the flush events were
missed because poll thread was not created. This change
fixes such issues.

Change-Id: I9478c208bc3d9d9b94a5e4b5148783e38a3d18aa
1 file changed