drm/amd/amdgpu: skip unload message in reset

This has been confirmed that unload message is not needed from SIENNA_CICHLID in reset.
Otherwise it will cause the fw wrong state after reset and no response for any messages.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>