drm/amdgpu/gfx6: drop gds_switch callback

GDS works differently on GFX6, plus the callback was
empty.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>