drm/amdgpu/gfx10: fix issues for suspend/resume

1). use PREEMPT_QUEUE instead of RESET_QUEUE for gfx ring disablement.
2). Need wait for unmapping queue done before continue execution.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Tianci Yin <tianci.yin@amd.com>
Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>