drm/msm/sde: do layer mixer assignment based on loop

For more general assignment of planes to layer mixers, cache LM
bounds at mode-set time along with split width point. Then, use
the bounds rectangles to do intersections with the plane
rectangles and use results for programming.

Change-Id: Ib4f18558dcf94a714684c9fb7e61ada58a070ce3
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>