Implement WaitVideoOutputStable method for VGA

It tries to probe the resolution from FPGA and waits until the
resolution is valid (no 0 reported) and not floating.

BUG=chromium:429931
TEST=Manually called some APIs, like DetectResolution, work fine.

Change-Id: Ie055ff052a97f95a0d68f051bed035437ddee040
Reviewed-on: https://chromium-review.googlesource.com/227785
Reviewed-by: Hung-ying Tyan <tyanh@chromium.org>
Commit-Queue: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Wai-Hong Tam <waihong@chromium.org>
1 file changed