drm/amdgpu: Cancel gfx off delay work when driver fini/suspend

there may be gfx off delay work pending when suspend/driver
unload, need to cancel them first.

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