drm/amdgpu: add vcn doorbell range function to nbio7.4 (v2)

To setup the aperture for VCN2.5

v2: setup vcn doorbells in vcn2.5 hw_init (Alex)

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>