drm/amdgpu: Fix an error message in rmmod

[why]
In rmmod procedure, kfd sends cp a dequeue request, but the
request does not get response, then an error message "cp
queue pipe 4 queue 0 preemption failed" printed.

[how]
Performing kfd suspending after disabling gfxoff can fix it.

Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>