drm/mediatek: Drop chain_mode_fixup call in mode_valid()

drm/mediatek: Drop chain_mode_fixup call in mode_valid()

The mode_valid implementation had a call to
drm_bridge_chain_mode_fixup() which would be wrong as the mode_valid is
not allowed to change anything - only to validate the mode.

As the next bridge is often/always a connector the call had no effect
anyway. So drop it.

From the git history I could see this call was included in the original
version of the driver so there was no help there to find out why it was
added in the first place. But a lot has changed since the initial driver
were added and is seems safe to remove the call now.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Cc: linux-mediatek@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240531-bridge_chain_mode-v1-1-8b49e36c5dd3@ravnborg.org/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>