drm/msm/sde: update mixercount check in writeback usecase

when writeback display width is greater than mixer
width use two layermixers, else single layermixer
is sufficient.

Change-Id: I9f1cc0fb839422d178c951cd197084f2a525a408
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>