drm/amdgpu/gfx8: fix resume of KIQ and KCQs

No need to reset the wptr and clear the rings.  The UNMAP_QUEUES
packet writes the current MQD state back the MQD on suspend,
so there is no need to reset it as well.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>