msm: mdss: optimize bw calculation when there is no blend

On mdp3 targets bg is not read in blit call when there is
no blending involved. Update the bandwidth calculation to
take care of this and reduce the bandwidth voting when
there is no blend involved.

Change-Id: I53399f87904106b9d84e62d38adcad151dc577a8
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>