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.

Signed-off-by: Peter Shkenev <mustela@erminea.space>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org