drm/amdgpu: change HWIP from UVD to VCN for VCN2.5

Rename hardware IP name from UVD to VCN to reduce confusion.
Hardware IP name UVD and VCN are equivalent for VCN2.5 asics.
Use name VCN for future VCN based asics.

V2: update description
V3: rebase

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: James Zhu <james.zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>