drm/dp_mst: Zero assigned PBN when releasing VCPI slots

Zero Port's PBN together with VCPI slots when releasing
allocated VCPI slots. That way when disabling the connector
it will not cause issues in drm_dp_mst_atomic_check verifying
branch bw limit.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: cd82d82c ("drm/dp_mst: Add branch bandwidth validation to MST atomic check")
Cc: <stable@vger.kernel.org> # v5.6+
Link: https://patchwork.freedesktop.org/patch/msgid/20200407160717.27976-1-mikita.lipski@amd.com