drm/amd/display: fix incorrect mpc_combine array size

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: fix incorrect mpc_combine array size [why] MAX_SURFACES is per stream, while MAX_PLANES is per asic. The mpc_combine is an array that records all the planes per asic. Therefore MAX_PLANES should be used as the array size. Using MAX_SURFACES causes array overflow when there are more than 3 planes. [how] Use the MAX_PLANES for the mpc_combine array size.