drm/amdgpu/gfx9.4.3: Explicitly halt MEC before init

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

Tested-by: Amber Lin <Amber.Lin@amd.com>
Acked-by: Amber Lin <Amber.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>