drm/msm/sde: assign relative mixer stages based on zpos

Assign mixer stages based on relative zpos ordering rather than
using the zpos property value directly. This avoids problems caused
by the zpos property value being greater than the available number
of stages.

Change-Id: I9f5a5a12ba5be49fbe1fbee78ddc7fadfeaedc9d
Signed-off-by: Clarence Ip <cip@codeaurora.org>