drm/amdgpu: fix VCN 3.1.2 firmware name

Drop the trailing vcn.

Fixes: afc2f276 ("drm/amdgpu/vcn: add vcn support for vcn 3.1.2")
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>