drm/amdgpu: modify sdma start sequence

should fist halt engine, and then doing the register
programing, and later unhalt engine, and finally run
ring_test.

this help fix reloading driver hang issue of SDMA
ring

original sequence is wrong for it programing engine
after unhalt, which will lead to fault behavior when
doing driver reloading after unloaded.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>