drm/amdgpu: drop redundant cancel_delayed_work_sync call

As those _sw_fini() APIs follow just after _suspend() APIs.
And the cancel_delayed_work_sync was already called in latter.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>