drm/meson: vclk: use the correct G12A frac max value

When calculating the HDMI PLL settings for a DMT mode PHY frequency,
use the correct max fractional PLL value for G12A VPU.

With this fix, we can finally setup the 1024x768-60 mode.

Fixes: 202b9808

 ("drm/meson: Add G12A Video Clock setup")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190828132311.23881-1-narmstrong@baylibre.com