drm/amdgpu/gfx8: Add serdes wait for idle in CGCG en/disable

Must wait for SERDES idle before exiting RLC SAFEMODE
Signed-off-by: NTom St Denis <tom.stdenis@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/gfx_v8_0.c

@@ -5805,6 +5805,8 @@ static void gfx_v8_0_update_coarse_grain_clock_gating(struct amdgpu_device *adev
 			WREG32(mmRLC_CGCG_CGLS_CTRL, data);
 	}
 
+	gfx_v8_0_wait_for_rlc_serdes(adev);
+
 	adev->gfx.rlc.funcs->exit_safe_mode(adev);
 }
 static int gfx_v8_0_update_gfx_clock_gating(struct amdgpu_device *adev,