drm/amdgpu: Ensure the DMA engine is deactivated during set ups

Setting the HALT bit of SDMA_F32_CNTL in all paths before programming
the ring buffer of the SDMA engine.

Signed-off-by: Haohui Mai <ricetons@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>