msm: mdss: Tune watermark and prefill BW based on minimum SMP bytes

Modify watermark levels and prefill BW calculation
to be set based on the minimum prefill buffer bytes from
the SMP. Add debugfs property to dynamically tune the minimum
bytes for debugging.

Change-Id: I3af1e3468eaab56d3a8eea5fe456d7af547d3244
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>