drm/msm/dp: check for invalid seqno before clearing a txmsg slot

Sometimes when the down reply and up request messages has to be
processed immediately, but the mst branch do not have enough tx
slots to transmit all the sideband ack messages, the driver fails
to allocate a slot for transmission and hence the sequence number
for the ack reply remains -1 or invalid. This is sometimes causing
a problem when the tx slot corresponding to the ack reply is
cleared without a proper sequence number check. These changes
ensure the sequence number is valid before clearing the tx slot.

Change-Id: I5a3655acd1f5045cedc75dfbd317867b3bc5cc8d
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>