drm/amdgpu: Consolidate VCN firmware setup code

Roughly the same code was present in all VCN versions.
Consolidate it into a single function.

v2: use AMDGPU_UCODE_ID_VCN + i, check if num_inst >= 2

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