drm/bridge: cdns-mhdp8546: Improve error reporting in remove callback

Replace the generic error message issued by the driver core when the remove
callback returns non-zero ("remove callback returned a non-zero value. This
will be ignored.") by a message that tells the actual problem.

Also simplify a bit by checking the return value of wait_event_timeout a
bit later.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231102165640.3307820-22-u.kleine-koenig@pengutronix.de