drm/amdgpu: check if hubbub is NULL in debugfs/amdgpu_dm_capabilities

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

drm/amdgpu: check if hubbub is NULL in debugfs/amdgpu_dm_capabilities

HUBBUB structure is not initialized on DCE hardware, so check if it is NULL to avoid null dereference while accessing amdgpu_dm_capabilities file in debugfs.