drm/mediatek: Return error if MDP RDMA failed to enable the clock

drm/mediatek: Return error if MDP RDMA failed to enable the clock

Return the result of clk_prepare_enable() instead of
always returns 0.

Fixes: f8946e2b6bb2 ("drm/mediatek: Add display MDP RDMA support for MT8195")
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20231214055847.4936-21-shawn.sung@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>