drm/amd/display: add DP sanity checks during enable stream

[why]
1. When HPD deassertion is pulled in the middle of
enabe stream link training, we will abort current training
and turn off PHY. This causes current link settings
to be zeroed this causes later stream enablement
sequence to fail as we prefer to carry on enablement
process despite of link training failure for SST.
2. When HPD is toggled after detection before before
the enable stream sequence as a result. There could be
a race condition where we could end up enable stream based
on the previous link even though the link is updated
after the HPD toggle. This causes an issue where our link
bandwidth is no longer enough to accommodate the timing
therefore causes us to oversubscribe MST payload time
slots. As discussed we decided to add basic sanity check
to make sure that our code can handle the oversubscription
failure silently without system hang.

[how]
1. Keep PHY powered on when HPD is deasserted during
enable stream and wait for the detection sequence to power
it off later.
2. Do not allocate payload if the required timeslot for
current timing is greater than 64 timeslots.

Reviewed-by: Aric Cyr <[email protected]>
Reviewed-by: George Shen <[email protected]>
Acked-by: Hamza Mahfooz <[email protected]>
Signed-off-by: Wenjing Liu <[email protected]>
Tested-by: Daniel Wheeler <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

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