drm/amdgpu: keep fbdev buffers pinned during suspend

Was dropped when we converted to the generic helpers.

Fixes: 087451f3 ("drm/amdgpu: use generic fb helpers instead of setting up AMD own's.")
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>