drm/hisilicon/hibmc: fix rare monitors cannot display problem

drm/hisilicon/hibmc: fix rare monitors cannot display problem

In some case, the dp link training success at 8.1Gbps, but the sink's
maximum supported rate is less than 8.1G. So change the default 8.1Gbps
link rate to the rate that reads from devices' capabilities.

Fixes: 54063d86e036 ("drm/hisilicon/hibmc: add dp link moduel in hibmc drivers")
Signed-off-by: Baihan Li <libaihan@huawei.com>
Signed-off-by: Yongbang Shi <shiyongbang@huawei.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250813094238.3722345-6-shiyongbang@huawei.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>