drm: bridge: it66121: Fix wait for DDC ready

The function it66121_wait_ddc_ready() would previously read the status
register until "true", which means it never actually polled anything and
would just read the register once.

Now, it will properly wait until the DDC hardware is ready or until it
reported an error.

The 'busy' variable was also renamed to 'error' since these bits are set
on error and not when the DDC hardware is busy.

Since the DDC ready function is now working properly, the msleep(20) can
be removed.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221214125821.12489-6-paul@crapouillou.net