msm: mdss: reduce mdp bandwidth for single pipe use case

For single layer use case, mdp prefill bandwidth request can be
reduced because mdp pipe operates like a fifo and there is no need to
fill all buffers before the active region.

Change-Id: Ic087f394b2551658543cff7f99789eace0388e07
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>