drm/amdgpu/gfx9: use common function to init cp fw

Use common function to init gfx v9 CP 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>