drm/amd/display: do not calculate DP2.0 SST payload when link is off

drm/amd/display: do not calculate DP2.0 SST payload when link is off

[Why & How]
There is a chance where the RX issues HPD deassert in the
middle of link training, this will cause our logic to
abort link training and turn off link. However our payload
allocation logic needs to use current link settings to
determine average time slot per MTP. This will need to
use current link bandwidth as divider. This causes divide
by zero error occasionally. The fix is to skip DP2.0 payload
allocation logic if current link is not in 128b/132b mode.

Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>