drm/amd/display: Add null check for top_pipe_to_program in commit_planes_for_stream

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Add null check for top_pipe_to_program in commit_planes_for_stream

This commit addresses a null pointer dereference issue in the

`commit_planes_for_stream` function at line 4140. The issue could occur

when `top_pipe_to_program` is null.

The fix adds a check to ensure `top_pipe_to_program` is not null before

accessing its stream_res. This prevents a null pointer dereference.

Reported by smatch:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:4140 commit_planes_for_stream() error: we previously assumed 'top_pipe_to_program' could be null (see line 3906)