drm/sde: keep cursor layer always on topmost blend stage

Even though mixer could support any blend stage for the HW
cursor layer, there are still some limitations on crossbar
which driver needs to keep it always on the topmost blend
stage.

Change-Id: I8f7b70fd3cead2f087dd0399f04cfb07278b7880
Signed-off-by: Jin Li <jinl@codeaurora.org>