drm/radeon: Enable sdma preemption

This patch adds to radeon the enablement of sdma preemption.
This is needed to support HWS of SDMA user-mode queues.

Signed-off-by: Ben Goz <ben.goz@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>