drm/mediatek: Add return value check when reading DPCD

drm/mediatek: Add return value check when reading DPCD

Check the return value of drm_dp_dpcd_readb() to confirm that
AUX communication is successful. To simplify the code, replace
drm_dp_dpcd_readb() and DP_GET_SINK_COUNT() with drm_dp_read_sink_count().

Fixes: f70ac097a2cf ("drm/mediatek: Add MT8195 Embedded DisplayPort driver")
Signed-off-by: Liankun Yang <liankun.yang@mediatek.com>
Reviewed-by: Guillaume Ranquet <granquet@baylibre.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20241218113448.2992-1-liankun.yang@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>