drm/amdgpu: rework psp firmware name

Use the new helper for deriving the fw name from
the IP version.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>