drm/amdgpu/display/mst: Fix mst_state->pbn_div and slot count assignments

Looks like I made a pretty big mistake here without noticing: it seems when
I moved the assignments of mst_state->pbn_div I completely missed the fact
that the reason for us calling drm_dp_mst_update_slots() earlier was to
account for the fact that we need to call this function using info from the
root MST connector, instead of just trying to do this from each MST
encoder's atomic check function. Otherwise, we end up filling out all of
DC's link information with zeroes.

So, let's restore that and hopefully fix this DSC regression.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/2171


Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Fixes: 4d07b0bc ("drm/display/dp_mst: Move all payload info into the atomic state")
Cc: stable@vger.kernel.org # 6.1
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Didier Raboud <odyx@debian.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>