drm: bridge: it66121: ->get_edid callback must not return err pointers

The drm stack does not expect error valued pointers for EDID anywhere.

Fixes: e6685650 ("drm: bridge: it66121: Set DDC preamble only once before reading EDID")
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Phong LE <ple@baylibre.com>
Cc: Neil Armstrong <neil.armstrong@linaro.org>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Robert Foss <rfoss@kernel.org>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Cc: <stable@vger.kernel.org> # v6.3+
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230914131159.2472513-1-jani.nikula@intel.com