msm: bam_dmux: Introduce watermarking in client transmit queue

BAM_DMUX does not put any restriction on the number of packets in
flight over the underlying BAM transport. This causes the bandwidth
to be not shared fairly among the clients. So introduce the concept
of watermarking in each client's transmit queue so that the clients
share the transport fairly and efficiently under high throughput
uplink scenarios

Change-Id: Ibf7e12b4cf13e826e7f83a41821f435cb7aa2cb0
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>