drm: bridge: it66121: Fix the register page length

Set the register page length or window length to
0x100 according to the documentation.

Fixes: 988156dc ("drm: bridge: add it66121 driver")
Signed-off-by: Nicolas Belin <nbelin@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220316135733.173950-3-nbelin@baylibre.com