drm/nouveau/dp: convert to use ERR_CAST()

As opposed to open-code, use ERR_CAST to clearly indicate that this is a
pointer to an error value and a type conversion is performed.

Signed-off-by: Zhang Enpei <zhang.enpei@zte.com.cn>
Link: https://lore.kernel.org/r/20250515201108576jof-gkjSxRfMaGDgKo-pc@zte.com.cn


Signed-off-by: Danilo Krummrich <dakr@kernel.org>