drm: bridge: it66121: Fix return value it66121_probe

drm: bridge: it66121: Fix return value it66121_probe

Currently it66121_probe returns -EPROBE_DEFER if the there is no remote
endpoint found in the device tree which doesn't seem helpful, since this
is not going to change later and it is never checked if the next bridge
has been initialized yet. It will fail in that case later while doing
drm_bridge_attach for the next bridge in it66121_bridge_attach.

Since the bindings documentation for it66121 bridge driver states
there has to be a remote endpoint defined, its safe to return -EINVAL
in that case.
This additonally adds a check, if the remote endpoint is enabled and
returns -EPROBE_DEFER, if the remote bridge hasn't been initialized
(yet).

Fixes: 988156dc2fc9 ("drm: bridge: add it66121 driver")
Signed-off-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210918140420.231346-1-knaerzche@gmail.com