drm/imx: ipuv3-plane: fix frame rate regression on non-plus i.MX6Q

Fix a regression introduced by 70e8a0c71e9 ("drm/imx: ipuv3-plane: add
function to query atomic update status") that halves the frame rate on
non-plus i.MX6Q, because the pending check always returns "pending"
even if an update is actually applied.
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