drm/amd/display: pbn_div need be updated for hotplug event

drm/amd/display: pbn_div need be updated for hotplug event

link_rate sometime will be changed when DP MST connector hotplug, so
pbn_div also need be updated; otherwise, it will mismatch with
link_rate, causes no output in external monitor.

Cc: stable@vger.kernel.org
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Jerry Zuo <jerry.zuo@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Wade Wang <wade.wang@hp.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>