drm/amdgpu: add fence suspend/resume functions

Added to:
- handle draining the ring on suspend
- properly enable/disable interrupts on suspend and resume

Fix breakages from:
commit 467ee3be53d240d08beed2e82a941e820c1ac323
Author: Chunming Zhou <david1.zhou@amd.com>
Date:   Mon Jun 1 14:14:32 2015 +0800

    drm/amdgpu: always enable EOP interrupt v2

Tested-by: Audrey Grodzovsky <audrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>