drm/rockchip: inno_hdmi: Remove useless mode_fixup

The mode_fixup implementation doesn't do anything, so we can simply
remove it.

Signed-off-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231222174220.55249-5-knaerzche@gmail.com