drm/amdgpu: add vcn ip dump ptr in vcn global struct

Add pointer to the vcn ip dump in the vcn global structure
to be accessible for all vcn version via global adev.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>