drm/amdgpu/gfx9: Explicitly halt CP before init

Need to make sure it's halted as we don't know what state
the GPU may have been left in previously.

Reviewed-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>