drm/amdgpu/sdma6: use common function to init sdma fw

Use common function to init sdma v6 firmware ucode.

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