drm/amdgpu: switch amdgpu_sdma_reset_engine to use the new sdma function pointers

Replace old callback mechanism with direct calls to stop/start functions.

Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>