drm/amdgpu: separate amdgpu_rlc into a single file

Separate the function and struct of RLC from the file of GFX.
Abstract the function of amdgpu_gfx_rlc_fini.

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>