drm/amdgpu: abstract the function of enter/exit safe mode for RLC

Abstract the function of amdgpu_gfx_rlc_enter/exit_safe_mode and some part of
rlc_init to improve the reusability of RLC.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>