drm/meson: hold 32 lines after vsync to give time for AFBC start

When using an AFBC encoded frame, the AFBC Decoder must be reset,
configured and enabled at each vsync IRQ.

To leave time for that, use the maximum lines hold time to give time
for AFBC setup and avoid visual glitches.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
[narmstrong: fix typo in commit log]
Link: https://patchwork.freedesktop.org/patch/msgid/20191021091509.3864-9-narmstrong@baylibre.com