drm/amdgpu: update SDMA V4 microcode init

Removed loading duplicate instances of SDMA FW for Arcturus.
We use a single image for all instances.

Signed-off-by: John Clements <john.clements@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>