Only enable firmware for the instance that is enabled.

v2: use AMDGPU_UCODE_ID_VCN + i

Fixes: 1b592d00 ("drm/amdgpu/vcn: remove manual instance setting")
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1743Reviewed-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>