drm/msm/mdp5: Stage border out on base stage if CRTC has 2 LMs

If a CRTC comprises of 2 LMs, it is mandatory to enable border out
and assign it to the base stage.

We had to enable border out also when the base plane wasn't fullscreen.
Club these checks and put them in a separate function called
get_start_stage() that returns the starting stage for assigning planes.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>