drm/amdgpu: add vcn ip dump support for vcn_v2_6

Add support for logging the registers in devcoredump
buffer for vcn_v2_6.

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>