drm/amd/display: Add null check for head_pipe in dcn201_acquire_free_pipe_for_layer

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

drm/amd/display: Add null check for head_pipe in dcn201_acquire_free_pipe_for_layer

This commit addresses a potential null pointer dereference issue in the

`dcn201_acquire_free_pipe_for_layer` function. The issue could occur

when `head_pipe` is null.

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

it. If `head_pipe` is null, the function returns NULL to prevent a

potential null pointer dereference.

Reported by smatch:

drivers/gpu/drm/amd/amdgpu/../display/dc/resource/dcn201/dcn201_resource.c:1016 dcn201_acquire_free_pipe_for_layer() error: we previously assumed 'head_pipe' could be null (see line 1010)