drm/amd/display: Add NULL check for clk_mgr and clk_mgr->funcs in dcn401_init_hw

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

drm/amd/display: Add NULL check for clk_mgr and clk_mgr->funcs in dcn401_init_hw

This commit addresses a potential null pointer dereference issue in the
`dcn401_init_hw` function. The issue could occur when `dc->clk_mgr` or
`dc->clk_mgr->funcs` is null.

The fix adds a check to ensure `dc->clk_mgr` and `dc->clk_mgr->funcs` is
not null before accessing its functions. This prevents a potential null
pointer dereference.

Reported by smatch:
drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn401/dcn401_hwseq.c:416 dcn401_init_hw() error: we previously assumed 'dc->clk_mgr' could be null (see line 225)