drm/amdgpu: Add support for resuming SDMA queues w/o HWS

Save wptr in hqd_sdma_destroy, restore it in hqd_sdma_load. Also
read updated wptr from user mode when resuming an SDMA queue.

Signed-off-by: Jay Cornwall <Jay.Cornwall@amd.com>
Signed-off-by: Yong Zhao <yong.zhao@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>