msm: mdss: fix check for hardware cursor mixer stage

Currently max target zorder is used to check where hw cursor needs to be
setup since it needs to be setup at highest mixer stage for target.
However max target zorder reflects the maximum number of layers
supported and not the actual mixer stage. Fix by doing proper check and
configuring right mixer stage for cursor.

Change-Id: I6dee63a852126110bdc4953239d5b92b77f12488
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>