drm/i915/mst: Do not retrain new links

The short pulse handler checks if channel equalization is okay and
goes onto retrain a link if there are active MST links. This retraining
path is not meant for new MST connections, but due to a bug elsewhere, if
active_mst_links is < 0 the boolean check for active_mst_links passes and
we proceed to retrain a new link. This results in a sequence of failed link
training attempts, most likely due to the hardware not setup for link
training at that point i.e., missing the DDI pre_enable sequence.

[   80.301272] [drm:intel_dp_check_mst_status] channel EQ not ok, retraining
[   80.301312] [drm:intel_ddi_prepare_link_retrain] *ERROR* Timeout waiting for DDI BUF C idle bit

The above error gives us a hint something went wrong before link
training started.

Check for a positive value of active_mst_links and throw in a warning for
invalid active_mst_links as debug aid.

Cc: Nathan Ciobanu <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Signed-off-by: Dhinakaran Pandiyan <[email protected]>
Tested-by: Nathan Ciobanu <[email protected]>
Reviewed-by: Rodrigo Vivi <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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