drm: meson: venc: set the correct macrovision max amplitude value

According to the register description of ENCI_MACV_MAX_AMP, the
macrovision max amplitude value should be:
- hdmi 480i => 0xb
- hdmi 576i => 0x7

The max value is 0x7ff (10 bits).

Signed-off-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/86mui782dt.fsf@baylibre.com