drm/amdgpu: add common gfx_ras_fini function

gfx_ras_fini can be shared among all generations of gfx

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>