drm/meson: hold fewer lines after vsync on Meson8/Meson8b/Meson8m2

On newer SoCs we need to hold more lines for AFBC support. See commit
24e0d405 ("drm/meson: hold 32 lines after vsync to give time for
AFBC start") for an in-depth explanation.

Testing on Meson8b shows that holding 31 lines after vsync causes screen
tearing, which is visible on both, CVBS and HDMI outputs for example
when running kmscube. Hold fewer lines after vsync (12 is taken from
Endless' 3.10 kernel sources for Meson8b) to fix this issue.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>