drm/msm/mdp4: fix mixer setup for multi-crtc + planes

On mdp4 there is a single global LAYERMIXER_IN_CFG register.  The
previous logic to share that between multiple crtcs didn't actually
handle plane-disable very well.  Easier just to look at all of the
crtcs each time.

Signed-off-by: Rob Clark <robdclark@gmail.com>