drm/amd/display: make dcn32_split_stream_for_mpc_or_odm static

It's not used outside of dcn32_fpu.c.

Fixes: 20dad381 ("drm/amd/display: Add a helper to map ODM/MPC/Multi-Plane resources")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>