drm/amdgpu: add vcn v2_6_0 ip headers (v3)

v1: Add vcn v2_6_0 register offset and
shift masks in header files (Hawking)
v2: Clean up vcn v2_6_0 registers (Alex)
v3: update registers (Alex)

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>