drm/amdgpu: Use single copy per SDMA instance type (v2)

v1: Only single copy per instance type is required for PSP. All instance types
use the same firmware copy. (Lijo)

v2: Apply the change into amdgpu_sdma_init_microcode() due to rebase. (Morris)

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>